Fig. 42.—The Cefn Cave, in Vale of Clwyd. (Trimmer.)a, Entrance;b, mud with pebbles and wood covered with stalagmite;c, mud, bones, and angular fragments of limestone;d, sand and silt, with fragments of marine shells;e, fissure;f, northern drift;g, cave cleared of mud;h, river Elwy, 100 feet below;i, limestone rock.
Fig. 42.—The Cefn Cave, in Vale of Clwyd. (Trimmer.)a, Entrance;b, mud with pebbles and wood covered with stalagmite;c, mud, bones, and angular fragments of limestone;d, sand and silt, with fragments of marine shells;e, fissure;f, northern drift;g, cave cleared of mud;h, river Elwy, 100 feet below;i, limestone rock.
“In the Vale of Clwyd an additional interest is imparted to the study of the drift from the circumstance that the remains of man have been found in deposits in caves sealed with drift-beds. The best example is the Cae Gwyn caves, in which flint implements and the bones and teeth of various extinct animals were found embedded in ‘cave-earth’which was overlaid by bedded deposits of shell-bearing drift, with erratics of the northern type.
“It has been supposed that the drift-deposits were marine accumulations; but it is inconceivable that the cave could ever have been subjected to wave-action without the complete scouring out of its contents.
“To resume the delineation of the limits of the great Irish Sea Glacier: From the Vale of Clwyd the boundary runs along the range of hills parallel to the estuary of the Dee at an altitude of about nine hundred feet. As it is traced to the southeast it gradually rises, until at Frondeg, a few miles to the northward of the embouchure of the Yale of Llangollen, it is at a height of 1,450 feet above sea-level. Thence it falls to 1,150 feet at Gloppa, three miles to the westward of Oswestry, and this is the most southerly point to which it has been definitely traced on the Welsh border, though scattered boulders of northern rocks are known to occur at Church Stretton.
“Along the line from the Vale of Clwyd to Oswestry the boundary is marked by a very striking series of moraine-mounds. They occur on the extreme summits of lofty hills in a country generally almost driftless, and their appearance is so unusual that one—Moel-y-crio—at least has been mistaken for an artificial tumulus. The limitation of the dispersal of northern erratics by these mounds is very clear and sharp; and Mackintosh, in describing those at Frondeg, remarked that, while no northern rocks extended to the westward of them, so no Welsh erratics could be found to cross the line to the eastward. There are Welsh erratics in the low grounds of Cheshire and Shropshire, but their distribution is sporadic, and will be explained in a subsequent section.
“Having thus followed around the edges of this glacier, it remains to describe its termination. It is clear that the ice must have forced its way over the low water-shed between the respective basins of the Dee and the Severn.So soon as this ridge (less than 500 feet above the sea) is crossed, we find the deposits laid down by the glacier change their character, and sands and gravels attain a great predominance.[BO]Near Bridgenorth, and, at other places, hills composed of such materials attain an altitude of 200 feet. From ShrewsburyviaBurton, and thence, in a semicircular sweep, through Bridgenorth and Enville, there is an immense concentration of boulders and pebbles, such as to justify the designation of a terminal moraine. To the southward, down the valley of the Severn, existing information points to the occurrence merely of such scattered pebbles as might have been carried down by floods. In the district lying outside this moraine there is a most interesting series of glacial deposits and of boulders of an entirely different character. (See map.)
[BO]Mackintosh, Q. J. G. S.
[BO]Mackintosh, Q. J. G. S.
“From the neighbourhood of Lichfield, through some of the suburbs of Birmingham, and over Frankley Hill and the Lickey Hills to Bromsgrove, there is a great accumulation of Welsh erratics, from the neighbourhood, probably, of Arenig Mawr.
“The late Professor Carvill Lewis suggested that these Arenig rocks might have been derived from some adjacent outcrop of Palæozoic rocks—a suggestion having its justification in the discoveries that had been made of Cumbrian rocks in the Midlands. To test the matter, an excavation was made at a point selected on Frankley Hill, and a genuine boulder-clay was found, containing erratics of the same type as those found upon the surface.
“The explanation has since been offered that this boulder-clay was a marine deposit laid down during a period of submerge nee.[BP]Apart from the difficulty that the boulder-clay displays none of the ordinary characteristics of a marine deposition, but possesses a structure, or ratherabsence of structure, in many respects quite inconsistent with such an origin, and contains no shells or other remains of marine creatures, it must be pointed out that no theory of marine notation will explain the distribution of the erratics, and especially their concentration in such numbers at a station sixty or seventy miles from their source.
[BP]Proceedings of the Birmingham Philosophical Society, vol. vi, Part I, p. 181.
[BP]Proceedings of the Birmingham Philosophical Society, vol. vi, Part I, p. 181.
“Upon the land-ice hypothesis this difficulty disappears. During the early stages of the Glacial period the Welsh ice had the whole of the Severn Valley at its mercy, and a great glacier was thrust down from Arenig, or some other point in central Wales, having aninitial direction, broadly speaking, from west to east. This glacier extended across the valley of the Severn, sweeping past the Wrekin, whence it carried blocks of the very characteristic rocks to be lodged as boulders near Lichfield; and it probably formed its terminal moraine along the line indicated. (See lozenge-shaped marks on the map.) As the ice in the north gathered volume it produced the great Irish Sea Glacier, which pressed inland and down the Vale of Severn in the manner I have described, and brushed the relatively small Welsh stream out of its path, and laid down its own terminal moraine in the space between the Welsh border and the Lickey Hills. It seems probable that the Welsh stream came mainly down the Vale of Llangollen, and thence to the Lickey Hills. Boulders of Welsh rocks occur in the intervening tract by ones and twos, with occasional large clusters, the preservation of any more connected trail being rendered impossible by the great discharge of water from the front of the Irish Sea Glacier, and the distributing action of the glacier itself.
“Within the area in England and Wales covered by the Irish Sea Glacier all the phenomena point to the action of land-ice, with the inevitable concomitants of subglacial streams, extra-morainic lakes, etc. There is nothing to suggest marine conditions in any form except the occurrence of shells or shell fragments; and these present somany features of association, condition, and position inconsistent with, what we should be led to expect from a study of recent marine life, that conchologists are unanimous in declaring that not one single group of them is on the site whereon the shells lived. It is a most significant fact—one out of a hundred which could be cited did space permit—that in the ten thousand square miles of, as it is supposed, recently elevated sea-bottom, not a single example of a bivalve shell with its valves in apposition has ever been found! Nor has a boulder or other stone been found encrusted with those ubiquitous marine parasites, the barnacles.
“The evidences of the action of land-ice within the area are everywhere apparent in the constancy of direction of— (1.) Striæ upon rock surfaces. (2.) The terminal curvature of rocks. (3.) The ‘pull-over’ of soft rocks. (4.) The transportal of local boulders. (5.) The orientation of the long axes of large boulders. (6.) The false bedding of sands and gravels. (7.) The elongation of drift-hills. (8.) The relations of ‘crag and tail.’ There is a similar general constancy, too, in the directions of the striæ upon large boulders. Upon the under side they run longitudinally from southeast (or thereabouts) to northwest, while upon the upper surface they originate at the opposite end, showing that the scratches on the under side were produced by the stone being dragged from northwest to southeast, while those on the top were the product of the passage of stone-laden ice over it in the same direction.
“Such an agreement cannot be fortuitous, but must be attributed to the operation of some agent acting in close parallelism over the whole area. To attribute such regularity to the action of marine currents is to ignore the most elementary principles of marine hydrology. Icebergs must, in the nature of things, be the most erratic of all agents, for the direction of drift is determined—amongother varying factors—by the draught of the berg. A mass of small draught will be carried by surface currents, while one of greater depth will be brought within the influence of under-currents; and hence it not infrequently happens that while floe-ice is drifting, say, to the southeast, giant bergs will go crashing through it to the northwest. There are tidal influences also to be reckoned with, and it is matter of common knowledge that flotsam and jetsam travel back and forth, as they are alternately affected by ebb and flood tide.
“Bearing these facts in mind, it is surely too much to expect that marine ice should transport boulders (how it picked up many of them also requires explanation) with such unfailing regularity that it can be said without challenge,[BQ]‘boulders in this district [South Lancashire and Cheshire] never occur to the north or west of the parent rock.’ The same rule applies without a single authentic exception to the whole area covered by the eastern branch of the Irish Sea Glacier; and hence it comes about that not a single boulder of Welsh rock has ever been recorded from Lancashire.
[BQ]Brit. Assoc. Report, 1890, p. 343.
[BQ]Brit. Assoc. Report, 1890, p. 343.
“The Solway Glacier.—The pressure which forced much of the Irish Sea ice against the Cumbrian coast-line caused, as has been described, a cleavage of the flow near Ravenglass, and, having followed the southerly branch to its termination in the midlands, the remaining moiety demands attention.
“The ‘easting’ motion carried it up the Solway Frith, its right flank spreading over the low plain of northern Cumberland, which it strewed with boulders of the well-known ‘syenite’ (granophyre) of Buttermere. When this ice reached the foot of the Cross Fell escarpment, it suffered a second bifurcation, one branch pushing to the eastward up the valley of the Irthing and over into Tyneside,and the other turning nearly due southward and forcing its way up the broad Vale of Eden.
“Under the pressure of an enormous head of ice, this stream rose from sea-level, turned back or incorporated the native Cumbrian Glacier which stood in its path, and, having arrived almost at the water-shed between the northern and the southern drainage, it swept round to the eastward and crossed over the Pennine water-shed; not, however, by the lowest pass, which is only some 1,400 feet above sea-level, but by the higher pass of Stainmoor, at altitudes ranging from 1,800 to 2,000 feet. The lower part of the course of this ice-flow is sufficiently well characterised by boulders of the granite of the neighbourhood of Dalbeattie in Galloway; but on its way up the Vale of Eden it gathered several very remarkable rocks and posted them as way-stones to mark its course. One of these rocks, the Permian Brockram, occurs nowherein situat altitudes exceeding 700 feet, yet in the course of its short transit it was lifted about a thousand feet above its source. The Shap granite (see radiant point on map) is on the northern side of the east and west water-sheds of the Lake District, and reaches its extreme elevation, (1,656 feet) on Wasdale Pike; yet boulders of it were carried over Stainmoor, at an altitude of 1,800 feet literally by tens of thousands.
“This Stainmoor Glacier passed directly over the Pennine chain, past the mouths of several valleys, and into Teesdale, which it descended and spread out in the low grounds beyond. Pursuing its easterly course, it abutted upon the lofty Cleveland Hills and separated into two streams, one of which went straight out to sea at Hartlepool, while the other turned to the southward and flowed down the Vale of York, being augmented on its way by tributary glaciers coming down Wensleydale. The final melting seems to have taken place somewhere a little to the southward of York; but boulders of Shap granite bywhich its extension is characterised have been found as far to the southward as Royston, near Barnsley.
“The other branch of the Solway Glacier—that which travelled due eastward—passed up the valley of the Irthing, and over into that of the Tyne, and out to sea at Tynemouth. It carried the Scottish granites with it, and tributary masses joined on either hand, bringing characteristic boulders with them.
“The fate of those elements of the Solway Frith Glacier which reached the sea is not left entirely to conjecture. The striated surfaces near the coast of Northumberland indicate a coastwise flow of ice from the northward—probably from the Frith of Forth—and the glaciers coming out from the Tyne and Tees were deflected to the southward.
“There is conclusive evidence that this ice rasped the cliffs of the Yorkshire coast and pressed up into some of the valleys. Where it passed the mouth of the Tees near Whitby it must have had a height of at least 800 feet, but farther down the coast it diminished in thickness. It nowhere extended inland more than a mile or two, and for the most part kept strictly to the coast-line. Along the whole coast are scattered erratics derived from Galloway and the places lying in the paths of the glaciers. In many places the cliffs exhibit signs of rough usage, the rocks being crumpled and distorted by the violent impact of the ice. At Filey Brigg a well-scratched surface has been discovered, the striation being from a few degrees east of north.
“At Speeton the evidence of ice-sheet or glacier-work is of the most striking character. On the top of the cliffs of Cretaceous strata a line of moraine-hills has been laid down, extending in wonderful perfection for a distance of six miles. They consist of a mixture of sand, gravel, and a species of clay-rubble, with occasional masses of true boulder-clay, the whole showing the arched bedding so characteristicof such accumulations. At the northerly end the moraine keeps close to the edge of the chalk cliffs, which are there 400 feet high, and the hills are frequently displayed in section; but as the elevation of the cliffs declines they fall back from the edge of the cliffs and run quite across the headland of Flamborough, and are again exposed in section in Bridlington Bay. One remarkable and significant fact is pointed out, namely, that behind this moraine, within half a mile and at a lower level, the country is almost absolutely devoid of any drift whatever.
Fig. 43.—Moraine between Speeton and Flamborough (Lamplugh).
Fig. 43.—Moraine between Speeton and Flamborough (Lamplugh).
“The interpretation of these phenomena is as follows: When the valley-glaciers reached the sea they suffered the deflection which has been mentioned, partly as the result of the interference of ice from the east of Scotland, but also influenced directly by the cause which operated upon the Scottish ice and gave direction to it—that is, the impact of a great glacier from Scandinavia, which almost filled the North Sea, and turned in the eastward-flowing ice upon the British coast.
“It is easy to see how this pressure must have forced the glacier-ice against the Yorkshire coast, but vertical chalk cliffs 400 feet in height are not readily surmounted by ice of any thickness, however great, and so it coasted along and discharged its lateral moraine upon the cliff-tops. As the cliffs diminished in height we find the moraine farther inland, and, as I have pointed out, the ice completely overrode Flamborough Head. Amongst the boulders at Flamborough are many of Shap granite, a few Galloway granites, a profusion of Carboniferous rocks, brought by the Tyne branch of the Sol way Glacier as well as by that of Stainmoor, and, besides many torn from the cliffs of Yorkshire, a few examples of unquestionable Scandinavian rocks, such as the well-knownRhomben-porphyr. It is important to note that about ten to twenty miles from the Yorkshire coast there is a tract of sea-bottom called by trawlers ‘the rough ground,’ in allusion to the fact that it is strewn with large boulders, amongst which are many of Shap granite. This probably represents a moraine of the Teesdale Glacier, laid down at a time when the Scandinavian Glacier was not at its greatest development.
“On the south side of Flamborough Head the ‘buried cliff’ previously alluded to occurs. The configuration of the country shows—and the conclusion is established by numerous deep-borings—that the preglacial coast-line takes a great sweep inland from here, and that the plain of Holderness is the result of the banking-up of an immense thickness of glacialdébris. In the whole countryreviewed, from Tynemouth to Bridlington, wherever the ice came on to the land from the seaward, it brought in shells and fragmentary patches of the sea-bottom involved in its ground moraine. Space does not permit of a detailed description of the several members of the Yorkshire Drift, and I pass on to deal in a general way with the glacial phenomena of the eastern side of England.
“The East Anglian Glacier.—The influence of the Scandinavian ice is clearly seen in the fact that the entire ice-movement down the east coast south of Bridlington was all from theseaward. Clays, sands, and gravels, the products of a continuous mass of land-ice coming from the northeast are spread over the whole country, from the Trent to the high grounds on the north of London overlooking the Thames.
“The line of extreme extension of these drift-deposits runs from Finchley (near London), in the south across Hertfordshire, through Cambridgeshire, with outlying patches at Gogmagog and near Buckingham, and northwestward over a large portion of Leicestershire into the upper waters of the Trent, embracing the elevated region of Palæozoic rocks at Charnwood Forest, near Leicester.
“Reserving the consideration of the very involved questions connected with the drifts of the upper part of the Trent Valley, I may pass on to join the phenomena of the southeastern counties with those at Flamborough Head. From Nottinghamshire the limits of the drift of the East Anglian Glacier seem to run in a direction nearly due west to east, for the great oolitic escarpment upon which Lincoln Cathedral is built is absolutely driftless to the northward of the breach about Sleaford. However, along the western flank of the oolitic range true boulder-clay occurs, bordering and doubtless underlying the great fen-tract of mid-Lincolnshire; and the great LincolnshireWolds appear to have been completely whelmed beneath the ice.
“The most remarkable of the deposits in this area is the Great Chalky Boulder-Clay, which consists of clay containing much ground-up chalk, and literally packed with well-striated boulders of chalk of all sizes, from minute pebbles up to blocks a foot or more in diameter. Associated with them are boulders of various foreign rocks, and many flints in a remarkably fresh condition, and still retaining the characteristic white coat, except where partially removed by glacial attrition.
“One of the perplexing features of the glacial phenomena in the eastern counties of England is the extension of true chalky boulder-clay to the north London heights at Finchley and elsewhere; for only the faintest traces are to be found in the gravel deposits of the Thames Valley of any wash from such a deposit, or from a glacier carrying such materials.
“It has been suggested that the deposit may have been laid down in an extra-morainic lake, or in an extension of the North. Sea, but these suggestions leave the difficulty just where it was. If a lake or sea could exist without shores, a glacier-stream might equally dispense with banks. Within the area of glaciation, defined above, abundant evidence of the action of land-ice is obtainable, though striated surfaces are extremely rare—a fact attributable to the softness of the chalk and clays which occupy almost the whole area. Well-striated surfaces are found on the harder rocks, as, for example, on the oolitic limestone at Dunston, near Lincoln.
“Mr. Skertchly has remarked that the proofs of the action of land-ice are irrefragable. The Great Chalky Boulder-Clay covers an area of 3,000 square miles, and attains an altitude of 500 feet above the sea-level, thus bespeaking, if the product of icebergs, ‘an extensive gathering-ground of chalk, having an elevation of more than 500 feet. But where is it? Certainly not in Western Europe, for the chalk does not attain so great an elevation except in a few isolated spots.’[BR]
[BR]Geikie’s Great Ice Age, p. 360.
[BR]Geikie’s Great Ice Age, p. 360.
Fig. 44.—Diagram-section near Cromer (Woodward). 6. Gravel and sand (Middle Glacial) resting on contorted drift (loam, sand, and marl, with large included boulders of chalk); 5. Cromer till: 4. Laminated clay and sands (Leda myalis bed); 3. Fresh-water loams and sands:3a. Black fresh-water bed of Runton (upper fresh-water bed); 2. Forest bed—laminated clays and sands, with roots anddébrisof wood, bones of mammalia, estuarine mollusca, etc., the upper part in places penetrated by rootlets (rootlet bed);2a. Weybourn crag; 1. Chalk with flints; * Large included boulder of chalk.
Fig. 44.—Diagram-section near Cromer (Woodward). 6. Gravel and sand (Middle Glacial) resting on contorted drift (loam, sand, and marl, with large included boulders of chalk); 5. Cromer till: 4. Laminated clay and sands (Leda myalis bed); 3. Fresh-water loams and sands:3a. Black fresh-water bed of Runton (upper fresh-water bed); 2. Forest bed—laminated clays and sands, with roots anddébrisof wood, bones of mammalia, estuarine mollusca, etc., the upper part in places penetrated by rootlets (rootlet bed);2a. Weybourn crag; 1. Chalk with flints; * Large included boulder of chalk.
“It has been further pointed out by Mr. Skertchly, that the condition of the flints in this deposit furnishes strong evidence that they could not have been carried by floating ice nor upon a glacier, for, in either of the latter events, there must have been some exposure to the weather, which, as he remarks, would have rendered them worthless to the makers of gun-flints, whereas they are now regularly collected for their use.
“The way in which the boulder-clay is related to the rocks upon which it rests is a conclusive condemnation of any theory of floating ice; for example, where it rests upon Oxford Clay, it contains the fossils characteristic of that formation, as it is largely made up of the clay itself. The exceptions to this rule are as suggestive as those cases which conform to it. Each outcrop yields material to the boulder-clay to the south westward, showing a pull-over from the northeast.
“One of the most remarkable features of the drift of this part of England is the inclusion of gigantic masses of rock transported for a short distance from their native outcrop, very often with so small a disturbance that they have been mapped asin situ. Examples of chalk-masses 800 feet in length, and of considerable breadth and thickness, have been observed in the cliffs near Cromer, in Norfolk, but they are by no means restricted to situations near the coast. One example is mentioned in which quarrying operations had been carried on for some years before any suspicion was aroused that it was merely an erratic. The huge boulders were probably dislodged from the parent rock by the thrust of a great glacier, which first crumbled the beds, then sheared off a prominent fold and carried it along. This explanation we owe to Mr. Clement Reid.[BS]The drift-deposits of this region frequently contain shells, but they rarely constitute what may be termed a consistent fauna, usually showing such an association as could only be found where some agent had been at work gathering together shells of different habitats and geological age.
[BS]See Geology of the Country around Cromer, and Geology of Holderness, Memoirs of Geological Survey of England and Wales.
[BS]See Geology of the Country around Cromer, and Geology of Holderness, Memoirs of Geological Survey of England and Wales.
Fig. 45.—Section at right angles to the cliff through the westerly chalk bluff at Trimingham, Norfolk, showing the manner in which chalk masses are incorporated into the till (Clement Reid). Scale, 250 fret to an inch. A. Level of low-water spring-tides; B. Chalk, with sandy bed at *; C. Forest-bed series, etc., seen in the cliffs a few yards north and south of this point; D. Cromer till, stiff lead-colored boulder-clay; E. Fine, chalky sands, much false-bedded; F. Contorted drift, brown bouldery-clay with marked bedding- or fluxion-structure; G. The bed, above the white line were seen and measured by more snow and measured by Mr. Reid; * Chalk seenin situon beach.“If the ice-sheet, instead of flowing over the beds, happens to plough into them or abut against them, it would bend up a boss of chalk, as at Beeston. A more extensive disturbance, like that at Trimingham drives before it a long ridge of the bads, and nips up the chalk, till, like a cloth creased by the sliding of a heavy book, it is folded into an inverted anticlinal. A slight increase of pressure, and the third stage is reached—the top of the anticlinal being entirely sheared off, the chalk boulder driven up an incline, and forced into the overlying boulder-clays.” (Clement Reid.)
Fig. 45.—Section at right angles to the cliff through the westerly chalk bluff at Trimingham, Norfolk, showing the manner in which chalk masses are incorporated into the till (Clement Reid). Scale, 250 fret to an inch. A. Level of low-water spring-tides; B. Chalk, with sandy bed at *; C. Forest-bed series, etc., seen in the cliffs a few yards north and south of this point; D. Cromer till, stiff lead-colored boulder-clay; E. Fine, chalky sands, much false-bedded; F. Contorted drift, brown bouldery-clay with marked bedding- or fluxion-structure; G. The bed, above the white line were seen and measured by more snow and measured by Mr. Reid; * Chalk seenin situon beach.“If the ice-sheet, instead of flowing over the beds, happens to plough into them or abut against them, it would bend up a boss of chalk, as at Beeston. A more extensive disturbance, like that at Trimingham drives before it a long ridge of the bads, and nips up the chalk, till, like a cloth creased by the sliding of a heavy book, it is folded into an inverted anticlinal. A slight increase of pressure, and the third stage is reached—the top of the anticlinal being entirely sheared off, the chalk boulder driven up an incline, and forced into the overlying boulder-clays.” (Clement Reid.)
Fig. 45.—Section at right angles to the cliff through the westerly chalk bluff at Trimingham, Norfolk, showing the manner in which chalk masses are incorporated into the till (Clement Reid). Scale, 250 fret to an inch. A. Level of low-water spring-tides; B. Chalk, with sandy bed at *; C. Forest-bed series, etc., seen in the cliffs a few yards north and south of this point; D. Cromer till, stiff lead-colored boulder-clay; E. Fine, chalky sands, much false-bedded; F. Contorted drift, brown bouldery-clay with marked bedding- or fluxion-structure; G. The bed, above the white line were seen and measured by more snow and measured by Mr. Reid; * Chalk seenin situon beach.
“If the ice-sheet, instead of flowing over the beds, happens to plough into them or abut against them, it would bend up a boss of chalk, as at Beeston. A more extensive disturbance, like that at Trimingham drives before it a long ridge of the bads, and nips up the chalk, till, like a cloth creased by the sliding of a heavy book, it is folded into an inverted anticlinal. A slight increase of pressure, and the third stage is reached—the top of the anticlinal being entirely sheared off, the chalk boulder driven up an incline, and forced into the overlying boulder-clays.” (Clement Reid.)
“Attempts have been made to correlate the deposits over the whole area, but with very indifferent success. A consideration of the consequences of the invasion of the country by an ice-stream from the northeast will prepare us for any conceivable complication of the deposits.
“The main movement was against the drainage of the country, so that the ice-front must have been frequently in water. There would be aqueous deposition and erosion; the kneading up of morainic matter into ground-moraine; irregularities of distribution and deposition due to ice floating in an extra-morainic lake; flood-washes at different points of overflow; and other confusing causes, which make it rather matter for surprise that any order whatever is traceable.
“I now turn to the valley of the Trent. We find that it occupies such a position that it would be exposed, successively or simultaneously, to the action of ice-streams of most diverse origin. I have shown that the area to the westward of Lichfield was invaded at one period by a Welsh glacier, and at a subsequent one by the Irish Sea Glacier, and both of these streams entered the valley of the Trent or some of its affluents. From the eastward, again, the great North Sea Glacier encroached in like manner, carrying the Great Chalky Boulder-Clay even into the drainage area of the westward-flowing rivers near Coventry.
“The glacial geology of the Trent Valley from Burton to Nottingham has been ably dealt with by Mr. R. M. Deeley,[BT]who recognises a succession which may be generalised as follows: (1.) A lower series containing rocks derived from the Pennine chain; (2.) A middle series containing rocks from the eastward (chalky boulder-clay, etc.); and (3.) An upper series with Pennine rocks. Mr. Deeley thinks the Penninedébrismay have been brought by glaciers flowing down the valleys of the Dove, the Wye, and the Derwent; but, while recognising the importance of the testimony adduced, especially that of the boulders, I am compelled to reserve judgment upon this point until something like moraines or other evidences of local glaciers can be shown in those valleys. In their upper parts there is not a sign of glaciation. Some of the deposits described must have been laid down by land-ice; while the conformation of the country shows that during some stages of glaciation a lake must have existed into which the different elements of the converging glaciers must have projected. This condition will account for the remarkable commingling of boulders observed in some of the deposits. Welsh, Cumbrian, and Scottish rocks occur in the western portion of the Trent Valley. The overflow of the extra-morainic lake would find its way into the valleys of the Avon and Severn, and may be taken to account for the abundance of flints in some of the gravels.
[BT]Quarterly Journal Geological Society, vol. xlii, p. 437.
[BT]Quarterly Journal Geological Society, vol. xlii, p. 437.
“The Isle of Man.—This little island in mid-seas constituted in the early stages of the Glacial epoch an independent centre of glaciation, and from some of its valleys ice-streams undoubtedly descended to the sea; but with the growth of the great Irish Sea Glacier the native ice was merged in the invading mass, and at the climax of the period the whole island was completely buried, even to its highest peak (Snae Fell, 2,054 feet), beneath the ice. Theeffects of this general glaciation are clearly seen in the mantle of unstratified drift material which overspread the hills; in themoutonnéeappearance of the entire island; and in the transport of boulders of local rocks. The striations upon rock surfaces show a constancy of direction in agreement with the boulder-transport which can be ascribed to no other agency than a great continuous sheet of such dimensions as to ignore minor hills and valleys.
“The disposition of the striæ is equally conclusive, for we find that on a stepped escarpment of limestone both the horizontal and the vertical faces are striated continuously and obliquely from the one on to the other, showing that the ice had a power of accommodating itself to the surface over which it passed that could not be displayed by floating ice. There is a remarkable fact concerning the distribution of boulders on this island which would strike the most superficial observers, namely, that foreign rocks are confined to the low grounds. It might be argued that the local ice always retained its individuality, and so kept the foreign ice with its characteristic boulders at bay. But, apart from thea prioriimprobability of so small a hill-cluster achieving what the Lake District could not accomplish, the fact that Snae Fell, an isolatedconicalhill, is swathed in drift from top to bottom, is quite conclusive that the foreign ice must have got in. Why, then, did it carry no stones with it? The following suggestion I make not without misgivings, though there are many facts to which I might appeal that seem strongly corroborative:
“The hilly axis of the island runs in a general northeast and southwest direction, and it rises from a great expanse of drift in the north with singular abruptness, some of the hills being almost inaccessible to a direct approach without actual climbing. I imagine that the ice which bore down upon the northern end of the islandwas, so far as its lower strata were concerned, unable to ascend so steep an acclivity, and was cleft, and flowed to right and left. The upper ice, being of ice-sheet origin, would be relatively clean, and this flowing straight over the top of the obstruction would glaciate the country with such material as was lying loose upon the ground or could be dislodged by mere pressure. It would appear from published descriptions that the Isle of Arran offers the same problem, and I would suggest the application of the same solution to it.
“Marine shells occur in the Manx drift, but only in such situations as were reached by the ice-laden with foreign stones. They present similar features of association of shells of different habitat, and perhaps of geological age, to those already referred to as being common characteristics of the shell-faunas of the drift of the mainland. Four extinct species of mollusca have been recognised by me in the Manx drift.
“The Manx drift is of great interest as showing, perhaps better than any locality yet studied, those features of the distribution of boulders of native rocks which attest so clearly the exclusive action of land-ice. There are in the island many highly characteristic igneous rocks, and I have found that boulders of these rocks never occur to the northward of the parent mass, and very rarely in any direction except to the southwest.
“Cumming observed in regard to one rock, the Foxdale granite, that whereas the highest point at which it occursin situwas 657 feet above sea-level, boulders of it occurred in profusion within 200 feet of the summit of South Barrule (1,585 feet), a hill two miles only, in a southwesterly direction, from the granite outcrop.
“They also occur on the summit of Cronk-na-Irrey-Lhaa, 1,449 feet above sea-level. The vertical uplift has been 728 and 792 feet respectively.
“In the low grounds of the north of the island a finelydeveloped terminal moraine extends in a great sweep so as to obstruct the drainage and convert thousands of acres of land into lake and morass, which is only now yielding to artificial drainage. Many fine examples of drumlin and esker mounds occur at low levels in different parts of the island; and it was remarked nearly fifty years ago by Cumming, that their long axes were parallel to the direction of ice-movement indicated by the striated surfaces and the transport of boulders.
“The foreign boulders are mainly from the granite mountains of Galloway, but there are many flints, presumably from Antrim, a very small number of Lake District rocks, and a remarkable rock containing the excessively rare variety of hornblende, Riebeckite. This has now been identified with a rock on Ailsa Crag, a tiny islet in the Frith of Clyde; and a Manx geologist, the Rev. S. N. Harrison, has discovered a single boulder of the highly characteristic pitchstone of Corriegills, in the Isle of Arran.
“The So-called Great Submergence.
“It may be convenient to adduce some additional facts which render the theory of a great submergence of the country south of the Cheviots untenable.
“The sole evidence upon which it rests is the occurrence of shells, mostly in an extremely fragmentary condition, in deposits occurring at various levels up to about 1,400 feet above sea-level: A little space may profitably be devoted to a criticism of this evidence.
“Moel Tryfaen(‘The Hill of the Three Rocks’).—This celebrated locality is on the first rise of the ground between the Menai Straits and the congeries of hills constituting ‘Snowdonia’; and when we look to the northward from the top of the hill (1,350 feet) we see the ground rising from the straits in a series of gentle undulations whose smooth contours would be found from a walk across thecountry to be due to the thick mask of glacial deposits which obliterates the harsher features of the solid rocks.
“The deposits on Moel Tryfaen are exposed in a slate-quarry on the northern aspect of the hill near the summit, and consist of two wedges of structureless boulder-clay, each thinning towards the top of the hill. The lower mass of clay, wherever it rests upon the rock, contains streaks and irregular patches of eccentric form, of sharp, perfectly angular fragments of slate; and the underlying rock may be seen to be crushed and broken, its cleavage-laminæ being thrust over from northwest to southeast—that is,up-hill. The famous ‘shell-bed’ is a thick series of sands and gravels interosculated with the clays on the slope of the hill, but occupying the entire section above the slate towards the top. The bedding shows unmistakable signs of the action of water, both regular stratification and false bedding being well displayed. The stones occurring in the clays are mainly if not entirely Welsh, including some from the interior of the country, and they are not infrequently of large size—two or three tons’ weight—and well scratched.
“The stones found in the sands and gravels include a great majority of local rocks, but besides these there have been recorded the following:
[BU]The altitude at which this rock occurs on Ailsa Craig has not been announced, so 1 have put it as the extreme height of the island.
[BU]The altitude at which this rock occurs on Ailsa Craig has not been announced, so 1 have put it as the extreme height of the island.
“The shells in the Moel Tryfaen deposit have been fully described, so far as the enumeration of species and relative frequency are concerned, but little has been said as to their absolute abundance and their condition. The shells are extremely rare, and daring a recent visit a party of five persons, in an assiduous search of about two hours, succeeded in findingfive whole shellsand about two ounces of fragments. The opportunities for collecting are as good as could be desired. The sections exposed have an aggregate length of about a quarter of a mile, with a height varying from ten to twenty feet of the shelly portion; and besides this there are immense spoil-banks, upon whose rain-washed slopes fossil-collecting can be carried on under the most favorable conditions.
“I would here remark, that the occurrence of small seams of shelly material of exceptional richness has impressed collectors with the idea that they were dealing with a veritable shell-bed, when the facts would bear a very different interpretation. A fictitious abundance is brought about by a process of what may be termed ‘concentration,’ by the action of a gently flowing current of water upon materials of different sizes and different specific gravities. Shells when but recently vacated consist of materials of rather high specific gravity, penetrated by pores containing animal matter, so that the density of the whole mass is far below that of rocks in general, and hence a current too feeble to move pebbles would yet carry shells. Illustrations of this process may be observed upon any shore in the concentration of fragments of coal, corks, or other light material.
“Regarding the interpretation of these facts: The commonly received idea is, that the beds were laid down in the sea during a period of submergence, and that the shells lived, not perhaps on the spot, but somewhere near, and that the terminal curvature of the slate was produced by the grounding of icebergs which also brought the boulders.But if this hypothesis were accepted, it would be necessary to invest the flotation of ice with a constancy of direction entirely at variance with observed facts, for the phenomena of terminal curvature is shown" with perfect persistence of direction wherever the boulder-clay rests upon the rock; and, further, there is the highly significant fact, that neither the sands and gravels nor the rock upon which they rest show any signs of disturbance or contortion, such as must have been produced if floating ice had been an operative agent.
“The uplift of foreign rocks is equally significant; and when we take into account the great distances from which they have been borne and the frequency with which such an operation must have been repeated, the inadequacy becomes apparent of Darwin’s ingenious suggestion, that it might have been effected by a succession of uplifts by shore-ice during a period of slow subsidence; while the character and abundance of the molluscan remains invest with a species of irony the application of the term ‘shell-bed’ to the deposit.
“I now turn to the alternative explanation (seeante,p. 145), viz., that the whole of the phenomena were produced by a mass of land-ice which was forced in upon Moel Tryfaen from the north or northwest, overpowering any Welsh ice which obstructed its course. This view is in harmony with the observations regarding the ‘terminal curvature’ of the slates, the occurrence of sharp angular chips of slate in the boulder-clay, and the coincidence of direction of these indications of movement with the carry of foreign stones. The few shells and shell-crumbs in the sands and gravels would, upon this hypothesis, be the infinitesimal relics of huge shell-banks in the Irish Sea which were destroyed by the glacier and in part incorporated in its ground-moraine or involved in the ice itself. The sands and gravels would represent the wash which would take place wherever, by the occurrence of a ‘nunatak’ or byapproach to the edge of the ice, water could have a free escape.
“Two principal objections have been urged to the land-ice explanation of the Moel Tryfaen deposits. An able critic asks, ‘Can, then, ice walk up-hill?’ To this we answer, Given a sufficient ‘head’ behind it, and ice can certainly achieve that feat, as everyroche moutonnéeproves. If it be granted that ice on the small scale can move up-hill, there is no logical halting-place between the uplift of ten or twenty feet to surmount aroche moutonnée, and an equally gradual elevation to the height of Moel Tryfaen. Furthermore, the inland ice of Greenland is known to extrude its ground-moraine on the ‘weather-side’ of the nunataks, and the same action would account for the material uplifted on Moel Tryfaen.
“The second objection brought forward was couched in somewhat these terms: ‘If the Lake District had its ice-sheet, surely Wales had one also. Could not Snowdonia protect the heart of its own domain?’ Of course, Wales had its ice-sheet, and the question so pointedly raised by the objector needs an answer; and though it is merely a question of how much force is requisite to overcome a certain resistance (both factors being unknown), still there are features in the case which render it specially interesting and at the same time comparatively easy of explanation. It seems rather like stating a paradox, yet the fact is, that it was the proximity of Snowdon which, in my opinion, enabled the foreign ice to invade Wales at that point.
“A glance at the map will show that the ‘radiant point’ of the Welsh ice was situated on or near Arenig Mawr, and that the great mass of Snowdon stands quite on the periphery of the mountainous regions of North Wales, so that it would oppose its bulk to fend off the native ice-sheet and prevent it from extending seaward in that direction.