Fig. 34. -- Diagrammatic representation of the effect of a hill on the edge of the ice.
After the ice has disappeared, the influence of the obstruction might be found in the disposition of the drift. If recession began during the first stage, that is, when the ice edge was separated into lobes, the margin of the drift should be lobate, and would loop back around the ridge from its advanced position on either side. If recession began during the second stage, that is, when the lobes had become confluent and completely surrounded the hill, adriftless areawould appear in the midst of drift. If recession began during the third stage, that is, after the ice had moved on over the obstruction, the evidence of the sequence might be obliterated; but if the ice moved but a short distance beyond the hill, the thinner ice over the hill would have advanced less far than the thicker ice on either side (Fig.35), and the margin ofthe drift would show a re-entrant pointing back toward the hill, though not reaching it. All these conditions are illustrated in the Devil's lake region.
Fig. 35. -- Same as Fig.34, when the ice has advanced farther.
The region under description is partly covered with drift, and partly free from it. The limit of the ice, at the time of its maximum expansion is well defined at many points, and the nature and position of the drift limit are so unique as to merit attention (see PlatesIIandXXXVII). They illustrate many of the principles already discussed.
The ice which covered the region was the western margin of the Green Bay lobe (Fig.36) of the last continental ice sheet. Its limit in this region is marked by a ridge-like accumulation of drift, theterminal moraine, which here has a general north-south direction. The regionmay have been affected by the ice of more than one epoch, but since the ice of the last epoch advanced as far to the west in this region as that of any earlier epoch, the moraine is on the border between the
Fig. 36. -- Map showing relations of lobes of ice during the Wisconsin ice epoch, to the driftless area.See larger image
glaciated country to the east, and the driftless area to the west (PlatesIandII). That part of the moraine which lies west of the Wisconsin river follows a somewhat sinuous course from Kilbourn City to a point a short distance north of Prairie du Sac. The departures fromthis general course are especially significant of the behavior of glacier ice.
In the great depression between the quartzite ranges, the moraine bends westward, showing that the ice advanced farther on the lowlands than on the ridges. As the moraine of this low area approaches the south range, it curves to the east. At the point southwest of Baraboo where the easterly curve begins to show itself, the moraine lies at the north base of the quartzite range; but as it is traced eastward, it is found to lie higher and higher on the slope of the range, until it reaches the crest nearly seven miles from the point where the eastward course was assumed. At this point it crosses the range, and, once across the crest, it turns promptly to the westward on the lower land to the south. Here the ice advanced up the valley between the East bluff (east of the lake) and the Devil's nose (PlateXXXVII), again illustrating the fact that lowlands favor ice advance. The valley between the Devil's nose and the East bluff is a narrow one, and the ice advanced through it nearly to the present site of the lake. Meanwhile the restraining influence of the "nose" was making itself felt, and the margin of the ice curved back from the bottom of the bluff near Kirkland, to the top of the bluff at the end of the nose. Here the edge of the ice crossed the point of the nose, and after rounding it, turned abruptly to the west. Thence its edge lay along the south slope of the ridge, descending from the crest of the ridge at the nose, to the base of the ridge two miles farther west. Here the ice reached its limit on the lowland, and its edge, as marked by the moraine, turned southward, reaching the Wisconsin river about a mile and a half above Prairie du Sac.
The course of the terminal moraine across the ridges is such as the margin of the ice would normally have when it advanced into a region of great relief. The great loop in the moraine with its eastern extremity atk, PlateXXXVII, is explained by the presence of the quartzite ridge which retarded the advancing ice while it moved forward on either side. The minor loop around the Devil's nose is explained in the same way. Both themain loop, and the smaller one on the nose, illustrate the point made on p.89.
The narrow and curious loop atm, is of a slightly different origin, though in principle the same. It is in the lee of a high point in the quartzite ridge. The ice surmounted this point, and descended its western slope; but the thickness of the ice passing over the summit was so slight that it advanced but a short distance down the slope before its force was exhausted, while the thicker ice on either side advanced farther before it was melted.
Before especial reference is made to the drift of this particular region, it will be well to consider the character of drift deposits in general. When the ice of the continental glacier began its motion, it carried none of the stony and earthy debris which constitute the drift. These materials were derived from the surface over which the ice moved.
From the method by which it was gathered, it is evident that the drift of any locality may contain fragments of rock of every variety which occurs along the route followed by the ice which reached that locality. Where the ice had moved far, and where there were frequent changes in the character of the rock constituting its bed, the variety of materials in the drift is great. The heterogeneity of the drift arising from the diverse nature of the rocks which contributed to it islithological heterogeneity—a term which implies the commingling of materials derived from different rock formations. Thus it is common to find pieces of sandstone, limestone, quartzite, granite, gneiss, schist, etc., intimately commingled in the drift, wherever the ice which produced it passed over formations of these several sorts of rock. Lithological heterogeneity is one of the notable characteristics of glacial formations.
Another characteristic of the drift is itsphysical heterogeneity. As first gathered from the bed of moving ice, some of the
WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXXV.
Cut in drift, showing its physical heterogeneity.See larger image
materials of the drift were fine and some coarse. The tendency of the ice in all cases was to reduce its load to a still finer condition. Some of the softer materials, such as soft shale, were crushed or ground to powder, forming what is known in common parlance as clay. Clayey (fine) material is likewise produced by the grinding action of ice-carried bowlders upon the rock-bed, and upon one another. Other sorts of rock, such as soft sandstone, were reduced to the physical condition of sand, instead of clay, and from sand to bowlders all grades of coarseness and fineness are represented in the glacial drift.
Since the ice does not assort the material which it carries, as water does, the clay, sand, gravel and bowlders will not, by the action of the ice, be separated from one another. They are therefore not stratified. As left by the ice, these physically heterogeneous materials are confusedly commingled. The finer parts constitute a matrix in which the coarser are embedded.
Physical heterogeneity (PlateXXXV), therefore, is another characteristic of glacial drift. It is not to be understood that the proportions of these various physical elements, clay, sand, gravel, and bowlders, are constant. Locally any one of them may predominate over any or all the others to any extent.
Since lithological and physical heterogeneity are characteristics of glacial drift, they together afford a criterion which is often of service in distinguishing glacial drift from other surface formations. It follows that this double heterogeneity constitutes a feature which can be utilized in determining the former extension of existing glaciers, as well as the former existence of glaciers where glaciers do not now exist.
Another characteristic of glacial drift, and one which clearly distinguishes it from all other formations with which it might be confounded, is easily understood from its method of formation. If the ice in its motion holds down rock debris upon the rock surface over which it passes with such pressure as to polish and striate the bed-rock, the material carried will itself suffer wear comparable to that which it inflicts. Thus the stones, large and small, of glacial drift, will be smoothed and striated.
This sort of wear on the transported blocks of rock, is effected both by the bed-rock reacting on the bowlders transported over it, and by bowlders acting on one another in and under the ice. The wear of bowlders by bowlders is effected wherever adjacent ones are carried along at different rates. Since the rate of motion of the ice is different in different parts of the glacier, the mutual abrasion of transported materials is a process constantly in operation. A large proportion of the transported stone and blocks of rock may thus eventually become striated.
From the nature of the wear to which the stones are subjected when carried in the base of the ice, it is easy to understand that their shapes must be different from those of water-worn materials. The latter are rolled over and over, and thus lose all their angles and assume a more or less rounded form. The former, held more or less firmly in the ice, and pressed against the underlying rock or rock debris as they are carried slowly forward, have their faces planed and striated. The planation and striation of a stone need not be confined to its under surface. On either side or above it other stones, moving at different rates, are made to abrade it, so that its top and sides may be planed and scored. If the ice-carried stones shift their positions, as they may under various circumstances, new faces will be worn. The new face thus planed off may meet those developed at an earlier time at sharp angles, altogether unlike anything which water-wear is capable of producing. The stone thus acted upon shows a surface bounded by planes and more or less beveled, instead of a rounded surface such as water wear produces. We find, then, in the shape of the bowlders and smaller stones of the drift, and in the markings upon their surfaces, additional criteria for the identification of glacier drift (PlateXXXVI).
The characteristics of glacial drift, so far as concerns its constitution, may then be enumerated as, (1) its lithological, and (2) physical heterogeneity; (3) the shapes, and (4) the markings of the stones of the drift. In structure, the drift which is strictly glacial, is unstratified.
In the broadest sense of the term, all deposits made by glacier
WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXXVI.
Glaciated stones, showing both form and striae. (Matz.)See larger image
ice aremoraines. Those made beneath the ice and back from its edge constitute theground moraine, and are distinguished from the considerable marginal accumulations which, under certain conditions, are accumulated at or near the margin. These marginal accumulations areterminal moraines. Associated with the moraines which are the deposits of the ice directly, there are considerable bodies of stratified gravel and sand, the structure of which shows that they were laid down by water. This is to be especially noted, since lack of stratification is popularly supposed to be the especial mark of the formations to which the ice gave rise.
These deposits of stratified drift lie partly beyond the terminal moraine, and partly within it. They often sustain very complicated relations both to the ground and terminal moraines.
The drift as a whole is therefore partly stratified and partly unstratified. Structurally the two types are thoroughly distinct, but their relations are often most complex, both horizontally and vertically. A fuller consideration of these relations will be found on a later page.
The ground moraine constitutes the great body of the glacial drift.Bowlder clay, a term descriptive of its constitution in some places, andtill, are other terms often applied to the ground moraine. The ground moraine consists of all the drift which lodged beneath the ice during its advance, all that was deposited back from its edge while its margin was farthest south, and most of that which was deposited while the ice was retreating. From this mode of origin it is readily seen that the ground moraine should be essentially as widespread as the ice itself. Locally, however, it failed of deposition. Since it constitutes the larger part of the drift, the characteristics already enumerated (p.95) as belonging to drift in general are the characteristics of the till. Wherever obstacles to the progress of the ice lay in its path, there was a chance that these obstacles, rising somewhat into thelower part of the ice, would constitute barriers against which debris in the lower part of the ice would lodge. It might happen also that the ice, under a given set of conditions favoring erosion, would gather a greater load of rock-debris than could be transported under the changed conditions into which its advance brought it. In this case, some part of the load would be dropped and over-ridden. Especially near the margin of the ice where its thickness was slight and diminishing, the ice must have found itself unable to carry forward the loads of debris which it had gathered farther back where its action was more vigorous. It will be readily seen that if not earlier deposited, all material gathered by the under surface of the ice would ultimately find itself at the edge of the glacier, for given time enough, ablation will waste all that part of the ice occupying the space between the original position of the debris, and the margin of the ice. Under the thinned margin of the ice, therefore, considerable accumulations of drift must have been taking place while the ice was advancing. While the edge of the ice sheet was advancing into territory before uninvaded, the material accumulated beneath its edge at one time, found itself much farther from the margin at another and later time. Under the more forcible ice action back from the margin, the earlier accumulations, made under the thin edge, were partially or wholly removed by the thicker ice of a later time, and carried down to or toward the new and more advanced margin. Here they were deposited, to be in turn disturbed and transported still farther by the farther advance of the ice.
Since in its final retreat the margin of the ice must have stood at all points once covered by it, these submarginal accumulations of drift must have been made over the whole country once covered by the ice. The deposits of drift made beneath the marginal part of the ice during its retreat, would either cover the deposits made under the body of the ice at an earlier time, or be left alongside them. The constitution of the two phases of till, that deposited during the advance of the ice, and that deposited during its retreat, is essentially the same, and there is nothing in their relative positions to sharply differentiate them.They are classed together assubglacial till.
Subglacial till was under the pressure of the overlying ice. In keeping with these conditions of accumulation, the till often possesses a firmness suggestive of great compression. Where its constitution is clayey it is often remarkably tough. Where this is the case, the quality here referred to has given rise to the suggestive name "hard pan." Where the constitution of the till is sandy, rather than clayey, this firmness and toughness are less developed, or may be altogether wanting, since sand cannot be compressed into coherent masses like clay.
Constitution.—The till is composed of the more or less comminuted materials derived from the land across which the ice passed. The soil and all the loose materials which covered the rock entered into its composition. Where the ice was thick and its action vigorous, it not only carried away the loose material which it found in its path, but, armed with this material, it abraded the underlying rock, wearing down its surface and detaching large and small blocks of rock from it. It follows that the constitution of the till at any point is dependent upon the nature of the soil and rock from which it was derived.
If sandstone be the formation which has contributed most largely to the till, the matrix of the till will be sandy. Where limestone instead of sandstone made the leading contribution to it, the till has a more earthy or clayey matrix. Any sort of rock which may be very generally reduced to a fine state of division under the mechanical action of the ice, will give rise to clayey till.
The nature and the number of the bowlders in the till, no less than the finer parts, depend on the character of the rock overridden. A hard and resistant rock, such as quartzite, will give rise to more bowlders in proportion to the total amount of material furnished to the ice, than will softer rock. Shale or soft sandstone, possessing relatively slight resistance, will be much more completely crushed. They will, therefore, yield proportionately fewer bowlders than harder formations, and moreof the finer constituents of till.
The bowlders taken up by the ice as it advanced over one sort of rock and another, possessed different degrees of resistance. The softer ones were worn to smaller dimensions or crushed with relative ease and speed. Bowlders of soft rock are, therefore, not commonly found in any abundance at great distances from their sources. The harder ones yielded less readily to abrasion, and were carried much farther before being destroyed, though even such must have suffered constant reduction in size during their subglacial journey. In general it is true that bowlders in the till, near their parent formations, are larger and less worn than those which have been transported great distances.
The ice which covered this region had come a great distance and had passed over rock formations of many kinds. The till therefore contains elements derived from various formations; that is, it is lithologically heterogeneous. This heterogeneity cannot fail to attract the attention of one examining any of the many exposures of drift about Baraboo at road gradings, or in the cuts along the railway. Among the stones in the drift at these exposures are limestone, sandstone, quartzite, diabase, gabbro, gneiss, granite, schist, and porphyry, together with pieces of flint and chert.
Such an array may be found at any of the exposures within the immediate vicinity of Devil's lake. To the north, and a few miles to the south of the Baraboo ranges, the quartzite from these bluffs, and the porphyry from the point markedhin PlateIIare wanting, though other varieties of porphyry are present. The ice moved in a general west-southwest direction in this region, and the quartzite in the drift, so far as derived from the local formation, is therefore restricted to a narrow belt.
The physical heterogeneity may be seen at all exposures, and is illustrated in PlateXXXV. The larger stones of the drift are usually of some hard variety of rock. Near the Baraboo ranges, the local quartzite often predominates among the bowlders, and since suchbowlders have not been carried far, they are often little worn. Away from the ranges, the bowlders are generally of some crystalline rock, such as granite and diabase. Bowlders of these sorts of rock are from a much more distant source, and are usually well worn.
In general the till of any locality is made up largely of material derived from the formations close at hand. This fact seems to afford sufficient warrant for the conclusion that a considerable amount of deposition must have gone on beneath the ice during its movement, even back from its margin. To take a concrete illustration, it would seem that the drift of southeastern Wisconsin should have had a larger contribution than it has of material derived from Canadian territory, if material once taken up by the ice was all or chiefly carried down to its thinned edge before deposition. The fact that so little of the drift came from these distant sources would seem to prove that a large part of the material moved by the ice, is moved a relatively short distance only. The ice must be conceived of as continually depositing parts of its load, and parts which it has carried but a short distance, as it takes up new material from the territory newly invaded.
In keeping with the character of till in general, that about Devil's lake was derived largely from the sandstone, limestone and quartzite of the immediate vicinity, while a much smaller part of it came from more distant sources. This is especially noticeable in the fine material, which is made up mostly of the comminuted products of the local rock.
Topography.—The topography of the ground moraine is in general the topography already described (p.85) in considering the modification of preglacial topography effected by ice deposition. As left by the ice, its surface was undulating. The undulations did not take the form of hills and ridges with intervening valleys, but of swells and depressions standing in no orderly relationship to one another. Undrained depressions are found in the ground moraine, but they are, as a rule, broader and shallower than the "kettles" common to terminal moraines.
It is in the broad, shallow depressions of the ground moraine that many of the lakes and more of the marshes of southeastern Wisconsin are located.
The rolling, undulating topography characteristic of ground moraines is well shown about the City of Baraboo and between that point and the lake, and at many less easily designated points about Merrimac.
In thickness the ground moraine reaches at least 160 feet, though its average is much less—too little to obliterate the greater topographic features of the rock beneath. It is, however, responsible for many of the details of the surface.
The marginal portion of the ice sheet was more heavily loaded—certainly more heavily loaded relative to its thickness—than any other. Toward its margin the thinned ice was constantly losing its transportive power, and at its edge this power was altogether gone. Since the ice was continually bringing drift down to this position and leaving it there, the rate of drift accumulation must have been greater, on the average, beneath the edge of the ice than elsewhere.
Whenever, at any stage in its history, the edge of the ice remained essentially constant in position for a long period of time, the corresponding submarginal accumulation of drift was great, and when the ice melted, the former site of the stationary edge would be marked by a broad ridge or belt of drift, thicker than that on either side. Such thickened belts of drift areterminal moraines. It will be seen that a terminal moraine does not necessarily mark the terminus of the ice at the time of its greatest advance, but rather its terminus at any time when its edge was stationary or nearly so.
From the conditions of their development it will be seen that these submarginal moraines may be made up of materials identical with those which constitute the ground moraine, and such is often the case. But water arising from the melting of the ice, played a much moreimportant role at its margin than farther back beneath it. One result of its greater activity may be seen in the greater coarseness which generally characterizes the material of the terminal moraine as compared with that of the adjacent ground moraine. This is partly because the water carried away such of the finer constituents as it was able to transport, leaving the coarser behind. Further evidence of the great activity of water near the margin of the ice is to be seen in the relatively large amount of assorted and stratified sand and gravel associated with the terminal moraine.
Such materials as were carried on the ice were dropped at its edge when the ice which bore them melted from beneath. If the surface of the ice carried many bowlders, many would be dropped along the line of its edge wherever it remained stationary for any considerable period of time. A terminal moraine therefore embraces (1) the thick belt of drift accumulated beneath the edge of the ice while it was stationary, or nearly so; and (2) such debris as was carried on the surface of the ice and dumped at its margin. In general the latter is relatively unimportant.
At various stages in its final retreat, the ice made more or less protracted halts. These halting places are marked by marginal moraines of greater or less size, depending on the duration of the stop, and the amount of load carried.
A terminal moraine is not the sharp and continuous ridge we are wont to think it. It is a belt of thick drift, rather than a ridge, though it is often somewhat ridge-like. In width, it varies from a fraction of a mile to several miles. In the region under consideration it is rarely more than fifty feet high, and rarely less than a half mile wide, and a ridge of this height and width is not a conspicuous topographic feature in a region where the relief is so great as that of the Devil's lake region.
Topography of terminal moraines.—The most distinctive feature of a terminal moraine is not its ridge-like character, but its peculiar topography. In general, it is marked by depressions without outlets, associated with hillocks and short ridges comparable in dimensions tothe depressions. Both elevations and depressions are, as a rule, more abrupt than in the ground moraine. In the depressions there are many marshes, bogs, ponds and small lakes. The shapes and the abundance of round and roundish hills have locally given rise to such names as "The Knobs," "Short Hills," etc. Elsewhere the moraine has been named the "Kettle Range" from the number of kettle-like depressions in its surface. It is to be kept in mind that it is the association of the "knobs" and "kettles," rather than either feature alone, which is the distinctive mark of terminal moraine topography.
Fig. 37. -- Sketch of terminal moraine topography, on the quartzite ridge east of Devil's lake. (Matz.)
The manner in which the topography of terminal moraines was developed is worthy of note. In the first place, the various parts of the ice margin carried unequal amounts of debris. This alone would have caused the moraine of any region to have been of unequal height and width at different points. In the second place, the margin of the ice, while maintaining the samegeneralposition during the making of a moraine, was yet subject to many minor oscillations. It doubtless receded to some slight extent because of increased melting during the summer, to advance again during the winter. In its recession, the ice margin probably did not remain exactly parallel to its former position. If some parts receded more than others, the details of the line of its margin may have been much changed during a temporary retreat. When the ice again advanced, its margin may have again changed its form in some slightmeasure, so as to be parallel neither with its former advanced position, nor with its position after its temporary retreat. With each successive oscillation of the edge, the details of the margin may have altered, and at each stage the marginal deposits corresponded with the edge. There might even be considerable changes in the edge of the ice without any general recession or advance, as existing glaciers show.
It was probably true of the margin of the American ice sheet, as of existing glaciers, that there were periods of years when the edge of the ice receded, followed by like periods when it remained stationary or nearly so, and these in turn followed by periods of advance. During any advance, the deposits made during the period of recession would be overridden and disturbed or destroyed.
If the ice were to retreat and advance repeatedly during a considerable period of time, always within narrow limits, and if during this oscillation the details of its margin were frequently changing, the result would be a complex or "tangle" of minor morainic ridges of variable heights and widths. Between and among the minor ridges there would be depressions of various sizes and shapes. Thus, it is conceived, many of the peculiar hillocks and hollows which characterize terminal moraines may have arisen.
Some of the depressions probably arose in another way. When the edge of the ice retreated, considerable detached masses of ice might be left beyond the main body. This might be buried by gravel and sand washed out from the moraine. On melting, the former sites of such blocks of ice would be marked by "kettles." In the marginal accumulations of drift as first deposited, considerable quantities of ice were doubtless left. When this melted, the drift settled and the unequal settling may have given rise to some of the topographic irregularities of the drift.
The terminal moraine about Devil's lake.—On the lower lands, the terminal moraine of the Devil's lake region has the features characteristic of terminal moraines in general. It is a belt of thickdrift varying in width from half a mile or less to three-quarters of a mile or more. Its surface is marked by numerous hills and short ridges, with intervening depressions or "kettles." Some of the depressions among the hills contain water, making ponds or marshes, though the rather loose texture of the drift of this region is not favorable to the retention of water. The moraine belt, as a whole, is higher than the land on either side. It is therefore somewhat ridge-like, and the small, short hills and ridges which mark its surface, are but constituent parts of the larger, broader ridge.
Approached from the west, that is from the driftless side, the moraine on the lower lands is a somewhat prominent topographic feature, often appearing as a ridge thirty, forty or even fifty feet in height. Approached from the opposite direction, that is, from the ground moraine, it is notably less prominent, and its inner limit wherever located, is more or less arbitrary.
Fig. 38. -- Cut through the terminal moraine just east of Kirkland, partially diagrammatic.See larger image
A deep, fresh railway cut in the moraine southeast of Devil's lake illustrates its complexity of structure, a complexity which is probably no greater than that at many other points where exposures are not seen. The section is represented in Fig.38. The stratified sand to the right retains even the ripple-marks which were developed when it was deposited. To the left, at the same level, there is a body oftill(unstratified drift), over which is a bed of stoneless and apparently structureless clay. In a depression just above the clay with till both to the right and left, is a body of loam which possesses thecharacteristics of normal loess. It also contains calcareous concretions, though no shells have been found. This occurrence of loess is the more noteworthy, since loess is rarely found in association with drift of the last glacial epoch.[7]
The moraine on the main quartzite range.—In tracing the moraine over the greater quartzite range, it is found to possess a unique feature in the form of a narrow but sharply defined ridge of drift, formed at the extreme margin of the ice at the time of its maximum advance. For fully eleven miles, with but one decided break, and two short stretches where its development is not strong, this unique marginal ridge separates the drift-covered country on the one hand, from the driftless area on the other. In its course the ridge lies now on slopes, and now on summits, but in both situations preserves its identity. Where it rests on a plain, or nearly plain surface, its width at base varies from six to fifteen rods, and its average height is from twenty to thirty feet. Its crest is narrow, often no more than a single rod. Where it lies on a slope, it is asymmetrical in cross section (see Fig.39), the shorter slope having a vertical
Fig. 39. -- Diagrammatic cross-section of the marginal ridge as it occurs on the south slope of the Devil's Nose. The slope below, though glaciated, is nearly free from drift.
range of ten to thirty-five feet, and its longer a range of forty to one hundred feet. This asymmetrical form persists throughout all that portion of the ridge which lies on an inclined surface, the slope of which does not correspond with the direction of the moraine. Where it lies on a flat surface, or an inclined surfacethe slope of which corresponds in direction with the course of the ridge itself, its cross section is more nearly symmetrical (see Fig.40). In all essential characteristics this marginal ridge corresponds with theEnd-Moräneof the Germans.
Fig. 40. -- Diagrammatic cross-section of the marginal ridge as it appears when its base is not a sloping surface.
For the sake of bringing out some of its especially significant features, the ridge may be traced in detail, commencing on the south side of the west range. Where the moraine leaves the lowlands south of the Devil's nose, and begins the ascent of the prominence, the marginal ridge first appears at about the 940-foot contour (f, PlateXXXVII). Though at first its development is not strong, few rods have been passed before its crest is fifteen to twenty feet above the driftless area immediately to the north (see Fig.39) and from forty to one hundred feet above its base to the south, down the slope. In general the ridge becomes more distinct with increasing elevation, and except for two or three narrow post-glacial erosion breaks, is continuous to the very summit at the end of the nose (g). The ridge in fact constitutes the uppermost forty or forty-five feet of the crest of the nose, which is the highest point of the west range within the area shown on the map. Throughout the whole of this course the marginal ridge lies on the south slope of the nose, and has the asymmetrical cross section shown in Fig.39. Above (north of) the ridge at most points not a bowlder of drift occurs. So sharply is its outer (north) margin defined, that at many points it is possible to locate it within the space of less than a yard.
At the crest of the nose (g) the marginal ridge, without a break, swings northward, and in less than a quarter of a mile turns again to the west. Bearing to the north it presently reaches (ath) the edge of the precipitous bluff, bordering the
WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXXVII.
Topographic map (contour interval 100 feet) of a small area about Devil's lake, taken from the Baraboo sheet of the United States Geological Survey. Each contour line connects points of the same elevation, and the figures upon them give the heights above sea level. Where contour lines lie close together, they indicate steep slopes.See larger image
great valley at the south end of the lake. Between the two arms of the loop thus formed, the surface of the nose is so nearly level that it could have offered no notable opposition to the progress of the ice, and yet it failed to be covered by it.
In the great valley between the nose and the east bluff, the marginal ridge does not appear. In the bottom of the valley the moraine takes on its normal form, and the slopes of the quartzite ridges on either hand are much too steep to allow any body of drift, or loose material of any sort, to lodge on them.
Ascending the east bluff a little east of the point where the drift ridge drops off the west bluff, the ridge is again found (ati) in characteristic development. For some distance it is located at the edge of the precipitous south face of the bluff. Farther on it bears to the north, and soon crosses a col (j) in the ridge, building it up many feet above the level of the bed-rock. From this point eastward for about three miles the marginal ridge is clearly defined, the slopes about equal on either side, and the crest as nearly even as the topography of the underlying surface permits. The topographic relations in this part of the course are shown in Fig.40.
Atk, this marginal ridge attains its maximum elevation, 1,620 feet. At this great elevation, the ridge turns sharply to the northwest at an angle of more than 90°. Following this direction for little more than half a mile, it turns to the west. At some points in this vicinity the ridge assumes the normal morainic habit, but this is true for short distances only. Farther west, atl, it turns abruptly to the northeast and is sharply defined. It here loops about a narrow area less than sixty rods wide, and over half a mile in length, the sharpest loop in its whole course. The driftless tract enclosed by the arms of this loop is lower than the drift ridge on either hand. The ice on either side would need to have advanced no more than thirty rods to have covered the whole of it.
From the minor loop just mentioned, the marginal ridge is continued westward, being well developed for about a mile and a half. At this point the moraine swings south to the north end of Devil's lake, loses the unique marginal ridge which has characterized its outer edgeacross the quartzite range for so many miles, and assumes the topography normal to terminal moraines. At no other point in the United States, so far as known to the writers, is there so sharply marked a marginal ridge associated with the terminal moraine, for so long a distance.
From PlateIIit will be seen that the moraine as a whole makes a great loop to the eastward in crossing the quartzite range. From the detailed description just given of the course of the marginal ridge, it will be seen that it has three distinct loops; one on the Devil's nose (west ofg, PlateXXXVII); one on the main ridge (west ofk) and a minor one on the north side of the last (southwest ofm). The first and third are but minor irregularities on the sides of the great loop, the head of which is atk.
The significant fact in connection with these irregularities in the margin of the moraine is that each loop stands in a definite relation to a prominence. The meaning of this relation is at once patent. The great quartzite range was a barrier to the advance of the ice. Acting as a wedge, it caused a re-entrant in the advancing margin of the glacier. The extent and position of the re-entrant is shown by the course of the moraine in PlateII. Thus the great loop in the moraine, the head of which is atk, PlateXXXVII, was caused by the quartzite range itself.
The minor loops on the sides of the major are to be explained on the same principle. Northeast of the minor loop on the north side of the larger one (m) there are two considerable hills, reaching an elevation of nearly 1,500 feet. Though the ice advancing from the east-northeast overrode them, they must have acted like a wedge, to divide it into lobes. The ice which reached their summits had spent its energy in so doing, and was unable to move forward down the slope ahead, and the thicker bodies of ice which passed on either side of them, failed to unite in their lee (compare Figs.34and35). The application of the same principle to the loop on the Devil's nose is evident.
Constitution of the marginal ridge.—The material in the marginal ridge, as seen where erosion has exposed it, is till, abnormal, if at all, only in the large percentage of widely transported bowlders whichit contains. This is especially true of the surface, where in some places 90 per cent. of the large bowlders are of very distant origin, and that in spite of the fact that the ice which deposited them had just risen up over a steep slope of quartzite, which could easily have yielded abundant bowlders. In other places the proportion of foreign bowlders is small, no more than one in ten. In general, however, bowlders of distant origin predominate over those derived close at hand.
The slope of the upper surface of the ice at the margin.—The marginal ridge on the south slope of Devil's nose leads to an inference of especial interest. Its course lies along the south slope of the nose, from its summit on the east to its base on the west. Throughout this course the ridge marks with exactness the position of the edge of the ice at the time of its maximum advance, and its crest must therefore represent the slope of the upper surface of the ice at its margin.
The western end of the ridge (f, PlateXXXVII) has an altitude of 940 feet, and its eastern end (g) is just above the 1,500-foot contour. The distance from the one point to the other is one and three-fourths miles, and the difference in elevation, 560 feet. These figures show that the slope of the ice along the south face of this bluff was about 320 feet per mile. This, so far as known, is the first determination of the slope of the edge of the continental ice sheetat its extreme margin. It is to be especially noted that these figures are for the extreme edge of the ice only. The angle of slope back from the edge was doubtless much less.
While it is true that glacier ice does not distinctly stratify the deposits which it makes, it is still true that a very large part of the drift for which the ice of the glacial period was directly or indirectly responsible is stratified. That this should be so is not strange when it is remembered that most of the ice was ultimately converted into running water, just as the glaciers of today are. The relatively small portion which disappeared by evaporation was probably more than counterbalanced, at least near the margin of the ice, by the rain which fell upon it.It cannot be considered an exaggeration, therefore, to say that the total amount of water which operated on the drift, first and last, was hardly less than the total amount of the ice itself. The drift deposited by the marginal part of the ice was affected during its deposition, not only by the water which arose from the melting of the ice which did the depositing, but by much water which arose from the melting of the ice far back from the margin. The general mobility of the water, as contrasted with ice, allowed it to concentrate its activities along those lines which favored its motion, so that different portions of the drift were not affected equally by the water of the melting ice.
All in all it will be seen that the water must have been a very important factor in the deposition of the drift, especially near the margin of the ice. But the ice sheet had a marginal belt throughout its whole history, and water must have been active and effective along this belt, not only during the decadence of the ice sheet, but during its growth as well. It is further to be noted that any region of drift stood good chance of being operated upon by the water after the ice had departed from it, so that in regions over which topography directed drainage after the withdrawal of the ice, the water had the last chance at the drift, and modified it in such a way and to such an extent as circumstances permitted.
Its origin.—There are various ways in which stratified drift may arise in connection with glacier deposits. It may come into existence by the operation of water alone; or by the co-operation of ice and water. Where water alone was immediately responsible for the deposition of stratified drift, the water concerned may have owed its origin to the melting ice, or it may have existed independently of the ice in the form of lakes. When the source of the water was the melting ice, the water may have been running, when it was actively concerned in the deposition of stratified drift; or it may have been standing (glacial lakes and ponds), when it was passively concerned. When ice co-operated with water in the development of stratified drift the ice was generally a passive partner.
Glacial drainage.—The body of an ice sheet during any glacial period is probably melting more or less at some horizons all the time, and at all horizons some of the time. Most of the water which is produced at the surface during the summer sinks beneath it. Some of it may congeal before it sinks far, but much of it reaches the bottom of the ice without refreezing. It is probable that melting is much more nearly continuous in the body of a moving ice sheet than at its surface, and that some of the water thus produced sinks to the bottom of the ice without refreezing. At the base of the ice, so long as it is in movement, there is doubtless more or less melting, due both to friction and to the heat received by conduction from the earth below. Thus in the ice and under the ice there must have been more or less water in motion throughout essentially all the history of an ice sheet.
If it be safe to base conclusions on the phenomena of existing glaciers, it may be assumed that the waters beneath the ice, and to a less extent the waters in the ice, organized themselves to a greater or less degree into streams. For longer or shorter distances these streams flowed in the ice or beneath it. Ultimately they escaped from its edge. The subglacial streams doubtless flowed, in part, in the valleys which affected the land surface beneath the ice, but they were probably not all in such positions.
The courses of well-defined subglacial streams were tunnels. The bases of the tunnels were of rock or drift, while the sides and tops were of ice. It will be seen, therefore, that their courses need not have corresponded with the courses of the valleys beneath the ice. They may sometimes have followed lines more or less independent of topography, much as water may be forced over elevations in closed tubes. It is not to be inferred, however, that the subglacial streams were altogether independent of the sub-ice topography. The tunnels in which the water ran probably had too many leaks to allow the water to be forced up over great elevations. This, at least, must have been the case where the ice was thin or affected by crevasses. Under such circumstances the topography of the land surface must have been the controlling elementin determining the course of the subglacial drainage.
When the streams issued from beneath the ice the conditions of flow were more or less radically changed, and from their point of issue they followed the usual laws governing river flow. If the streams entered static water as they issued from the ice, and this was true where the ice edge reached the sea or a lake, the static water modified the results which the flowing waters would otherwise have produced.
Stages in the history of an ice sheet.—The history of an ice sheet which no longer exists involves at least two distinct stages. These are (1) the period of growth, and (2) the period of decadence. If the latter does not begin as soon as the former is complete, an intervening stage, representing the period of maximum ice extension, must be recognized. In the case of the ice sheets of the glacial period, each of these stages was probably more or less complex. The general period of growth of each ice sheet is believed to have been marked by temporary, but by more or less extensive intervals of decadence, while during the general period of decadence, it is probable that the ice was subject to temporary, but to more or less extensive intervals of recrudescence. For the sake of simplicity, the effects of these oscillations of the edge of the ice will be neglected at the outset, and the work of the water accompanying the two or three principal stages of an ice sheet's history will be outlined as if interruptions in the advance and in the retreat, respectively, had not occurred.
As they now exist, the deposits of stratified drift made at the edge of the ice or beyond it during the period of its maximum extension present the simplest, and at the same time most sharply defined phenomena, and are therefore considered first.
The deposits made by the water at the time of the maximum extension of the ice and during its final retreat, were never disturbed by subsequent glacier action. So far as not destroyed by subsequent erosion, they still retain the form and structure which they had at the outset. Such drift deposits, because they lie at the surface, and because they are more or less distinct topographically as well as structurally, are better known than the stratified drift of other stages of an ice sheet's history. Of stratified drift made during the maximum extension of the ice, and during its final retreat, there are several types.
A. At the edge of ice, on land.—If the subglacial streams flowed under "head," the pressure was relieved when they escaped from the ice. With this relief, there was diminution of velocity. With the diminution of velocity, deposition of load would be likely to take place. Since these changes would be likely to occur at the immediate edge of the ice, one class of stratified drift deposits would be made in this position, in immediate contact with the edge of the ice, and their form would be influenced by it. At the stationary margin of an ice sheet, therefore, at the time of its maximum advance, ice and water must have co-operated to bring into existence considerable quantities of stratified drift.
The edge of the ice was probably ragged, as the ends of glaciers are today, and as the waters issued from beneath it, they must frequently have left considerable quantities of such debris as they were carrying, against its irregular margin, and in its re-entrant angles and marginal crevasses. When the ice against which this debris was first lodged melted, the marginal accumulations of gravel and sand often assumed the form ofkames. A typical kame is a hill, hillock, or less commonly a short ridge of stratified drift; but several or many are often associated, giving rise to groups and areas of kames. Kames are oftenassociated with terminal moraines, a relation which emphasizes the fact of their marginal origin.
So far as the superficial streams which flowed to the edge of the ice carried debris, this was subject to deposition as the streams descended from the ice. Such drift would tend to increase the body of marginal stratified drift from subglacial sources.
Marginal accumulations of stratified drift, made by the co-operation of running water and ice, must have had their most extensive development, other things being equal, where the margin of the ice was longest in one position, and where the streams were heavily loaded. The deposits made by water at the edge of the ice differ from those of the next class—made beyond the edge of the ice—in that they were influenced in their disposition and present topography, by the presence of ice.
In the Devil's lake region isolated and well-defined kames are not of common occurrence. There are, however, at many points hills which have something of a kame-like character. There is such a hill a mile southeast of the Court house at Baraboo, at the point markedp, PlateXXXVII. In this hill there are good exposures which show its structure. There are many hillocks of a general kame-like habit associated with the terminal moraine south of the main quartzite range, and north of the Wisconsin river. Many of them occur somewhat within the terminal moraine a few miles northwest of Merrimac.
B. Beyond the edge of the ice, on land.—As the waters escaping from the ice flowed farther, deposits of stratified drift were made quite beyond the edge of the ice. The forms assumed by such deposits are various, and depended on various conditions. Where the waters issuing from the edge of the ice found themselves concentrated in valleys, and where they possessed sufficient load, and not too great velocity, they aggraded the valleys through which they flowed, developing fluvial plains of gravel and sand, which often extended far beyond the ice. Such fluvial plains of gravel and sand constitute thevalley trainswhich extend beyond the unstratified glacial drift in many of the valleys ofthe United States. They are found especially in the valleys leading out from the stouter terminal moraines of late glacial age. From these moraines, the more extensive valley trains take their origin, thus emphasizing the fact that they are deposits made by water beyond a stationary ice margin. Valley trains have all the characteristics of alluvial plains built by rapid waters carrying heavy loads of detritus. Now and then their surfaces present slight variations from planeness, but they are minor. Like all plains of similar origin they decline gradually, and with diminishing gradient, down stream. They are of coarser material near their sources, and of finer material farther away. Valley trains constitute a distinct topographic as well as genetic type.
A perfect example of a valley train does not occur within the region here discussed. There is such a train starting at the moraine where it crosses the Wisconsin river above Prairie du Sac, and extending down that valley to the Mississippi, but at its head this valley train is wide and has the appearance of an overwash plain, rather than a valley train. Farther from the moraine, however, it narrows, and assumes the normal characteristics of a valley train. It is the gravel and sand of this formation which underlies Sauk Prairie, and its topographic continuation to the westward.
Where the subglacial streams did not follow subglacial valleys, they did not always find valleys when they issued from the ice. Under such circumstances, each heavily loaded stream coming out from beneath the ice must have tended to develop a plain of stratified material near its point of issue—a sort of alluvial fan. Where several such streams came out from beneath the ice near one another, their several plains, or fans, were likely to become continuous by lateral growth. Such border plains of stratified drift differ from valley trains particularly (1) in being much less elongate in the direction of drainage; (2) in being much more extended parallel to the margin of the ice; and (3) in not being confined to valleys. Such plains stood an especially good chance of development where the edge of the ice remained constant for aconsiderable period of time, for it was under such conditions that the issuing waters had opportunity to do much work. Thus arose the type of stratified drift variously known asoverwash plains,outwash plains,morainic plains, andmorainic aprons. These plains sometimes skirt the moraine for many miles at a stretch.
Overwash plains may sometimes depart from planeness by taking on some measure of undulation, of the sag and swell (kame) type, especially near their moraine edges. The same is often true of the heads of valley trains. The heads of valley trains and the inner edges of overwash plains, it is to be noted, occupy the general position in which kames are likely to be formed, and the undulations which often affect these parts of the trains and plains, respectively, are probably to be attributed to the influence of the ice itself. Valley trains and overwash plains, therefore, at their upper ends and edges respectively, may take on some of the features of kames. Indeed, either may head in a kame area.
Good examples of overwash or outwash plains may be seen at various points in the vicinity of Baraboo. The plain west of the moraine just south of the main quartzite ridge has been referred to under valley trains. In Sauk Prairie, however, its characteristics are those of an outwash plain, rather than those of a valley train.