6. Geology.
Geology is the study of the rocks or the substances of which the earthy crust of a district is composed. Rocks are of two sorts: (1) those due to the action of heat, called igneous, (2) those formed and deposited by water, called aqueous. When the earth was a molten ball, it cooled at the surface, but every now and again liquid portions were ejected from cracks and weak places. The same process is seen in the eruptions of Mount Vesuvius, which sends out streams of liquid lava that gradually cools and forms hard rock. Such areigneousrocks. But all the forces of nature are constantly at work disintegrating the solid land; frost, rain, the action of rivers and the atmosphere wear down the rocks; and the tiny particles are carried during floods to the sea, where they are deposited as mud or sand-beds laid flat one on the top of the other like sheets of paper. These are _aqueous_ rocks. The layers are afterwards apt to be tilted up on end or at various angles owing to the contortions of the earth’s crust, through pressure in particular directions. When so tilted they may rise above water and immediately the same process that made them now begins to unmake them. They too may in time be so worn away that only fragments of them are left whereby we may interpret their history.
Loch of Skene
Loch of Skene
To these may be added a third kind of rock calledmetamorphic, or rocks so altered by the heat and pressure of other rocks intruding upon them, that they lose their original character and become metamorphosed. They may be either sedimentary, laid down originally by water, or they may be igneous, but in both cases they are entirely changed or modified in appearance and structure by the treatment they have suffered.
The geology of Aberdeenshire is almost entirely concerned with igneous and metamorphic rocks. The whole backbone of the county is granite which has to some extent been rubbed smooth by glacial action; but in a great part of the county the granite gives place to metamorphic rocks, gneiss, schist, and quartzite. A young geologist viewing a deep cutting in the soil about Aberdeen finds that the material consists of layers of sand, gravel, clay, which are loosely piled together all the way down to the solid granite. This is the glacial drift, or boulder clay, a much later formation than the granite and a legacy of what is called the great Ice Age. Far back in a time before the dawn of history all the north-east of Scotland was buried deep under a vast snow-sheet. The snow consolidated into glaciers just as in Switzerland to-day, and the glaciers thus formed worked their way down the valleys, carrying a great quantity of loose material along with them. When a warmer time came, the ice melted and all the sand and boulders mixed up in the ice were liberated and sank as loose deposits on the land. This is the boulder clay which in and around Aberdeen is the usual subsoil. It consists of rough,half-rounded pebbles, large and small, of clay, sand, and shingle, and makes a very cold and unkindly soil, being difficult to drain properly and slow to take in warmth.
Below this boulder clay are the fundamental rocks. At Aberdeen these are pure granite; but in other parts of the county they are, as we have said, metamorphic, that is, they have been altered by powerful forces, heat and pressure. Whether they were originally sedimentary, before they were altered, is doubtful; some geologists think the crystalline rocks round Fraserburgh and Peterhead were aqueous. Mormond Hill was once a sandstone, and the schists of Cruden Bay and Collieston were clay. The same beds traced to the south are found to pass gradually into sedimentary rocks that are little altered. Whether they were aqueous or igneous originally, they have to-day lost all their original character. No fossils are found in them. These rocks are the oldest and lowest in Aberdeenshire. After their formation, they were invaded from below by intrusive masses of molten igneous rock, which in many parts of the county is now near the surface. This is the granite already referred to. Its presence throughout the county has materially influenced the character and the industry of the people.
Wherever granite enters, it tears its irregular way through the opposing rocks, and sends veins through cracks where such occur. The result of its forcible entrance in a molten condition is that the contiguous rocks are melted, blistered, and baked by the intrusive matter. Why granite should differ from the lava wesee exuding from active volcanoes is explained by the fact that it is formed deep below the surface where there is no outlet for its gases. It cools slowly and under great pressure and this gives it its special character. If found, therefore, at the surface, as it is in Aberdeen, this is because the rocks once high above it, concealing its presence have been worn away, which gives some idea of the great age of the district. One large granitic mass is at Peterhead, where it covers an area of 46 square miles, and forms the rocky coast for eight miles; but the whole valley of the Dee as far as Ben Macdhui, and a great part of Donside, consist of this intrusive granite. It varies in colour and quality, being in some districts reddish in tint as at Sterling Hill near Peterhead, at Hill of Fare, and Corennie; in other parts it is light grey in various shades.
The succession in the order of sedimentary rocks is definitely settled, and although this has little application to Aberdeenshire, an outline may be given. The oldest are the Palaeozoic which includes—in order of age—
Cambrian,Silurian,Old Red Sandstone or Devonian,Carboniferous,Permian.
Of these the only one represented in Aberdeenshire is the Old Red Sandstone, which occupies a considerable strip on the coast from Aberdour to Gardenstown, and runs inland to Fyvie and Auchterless and even as far as Kildrummy and Auchindoir. The deposit is 1300 feetthick. A visitor to the town of Turriff is struck by the red colour of many of the houses there, a most unusual variant upon the blue-grey whinstone of the surrounding districts. The explanation is that a convenient quarry of Old Red Sandstone exists between Turriff and Cuminestown. Kildrummy Castle, one of the finest and most ancient ruins in the county, is not like the majority of the old castles built of granite but of a sandstone in the vicinity. The same band extends across country to Auchindoir, where it is still quarried.
The next geological group of Rocks, the Secondary or Mesozoic, includes—in order of age—
Triassic,Jurassic,Cretaceous.
These are not at all or but barely represented. A patch of clay at Plaidy, which was laid bare in cutting the railway track, belongs to the Jurassic system and contains ammonites and other fossils characteristic of that period. Over a ridge of high ground stretching from Sterling Hill south-eastwards are found numbers of rolled flints belonging to the Cretaceous or chalk period, but the probability is that they have been transported from elsewhere by moving ice and are not in their natural place.
The Tertiary epoch is just as meagrely represented as the Secondary. Yet this is the period which in other parts of the world possesses records of the most ample kind. The Alps, the Caucasus, the Himalayas were all upheaved in Tertiary times; but of any correspondingactivity in the north-east of Scotland, there is no trace. It is only when the Tertiary merges in the Quaternary period that the history is resumed. The deposits of the Ice Age, when Scotland was under the grip of an arctic climate, are much in evidence all over the county and have already been referred to. It is necessary to treat the subject in some detail.
During the glacial period, the snow and ice accumulated on the west side of the country, and overflowed into Aberdeenshire. There were several invasions owing to the recurrence of periods of more genial temperature when the ice-sheet dwindled. One of the earlier inroads probably brought with it the chalk flints now found west of Buchan Ness; another brought boulders from the district of Moray. South of Peterhead a drift of a different character took place. Most of Slains and Cruden as well as Ellon, Foveran, and Belhelvie are covered with a reddish clay with round red pebbles like those of the Old Red Sandstone. This points to an invasion of the ice-sheet from Kincardine, where such deposits are rife. Dark blue clay came from the west, red clay from the south, and in some parts they met and intermixed as at St Fergus. A probable third source of glacial remains is Scandinavia. In the Ice Age Britain was part of the continental mainland, the shallow North Sea having been formed at a subsequent period. The low-lying land at the north-east of the county was the hollow to which the glaciers gravitated from west and south and east, leaving theirdébrison the surface when the ice disappeared. So much is this a feature of Buchan that one well-knowngeologist has humorously described it as the riddling heap of creation.
Both the red and the blue clay are often buried under the coarse earthy matter and rough stones that formed the residuum of the last sheet of ice. This has greatly increased the difficulty of clearing the land for cultivation. Moreover a clay subsoil of this kind, which forms a hard bottom pan that water cannot percolate through, is not conducive to successful farming. Drainage is difficult but absolutely necessary before good crops will be produced. Both difficulties have been successfully overcome by the Aberdeenshire agriculturist, but only by dint of great expenditure of time and labour and money.
The district of the clays is associated with peat beds. There is peat, or rather there was once peat all over Aberdeenshire, but the depth and extent of the beds are greatest where the clay bottom exists. A climate that is moist without being too cold favours the growth of peat and the Buchan district, projecting so far into the North Sea and being subject to somewhat less sunshine than other parts of the county, provides the favouring conditions. The rainfall is only moderate but it is distributed at frequent intervals, and the clay bottom helps to retain the moisture and thus promotes the growth of those mosses which after many years become beds of peat. These peat beds for long provided the fuel of the population. In recent years they are all but exhausted, and the facility with which coals are transported by sea and by rail is gradually putting an end to the “casting” and drying of peats.
Moraines of rough gravel—the wreckage of dwindling glaciers—are found in various parts of the Dee valley. The soil of Deeside has little intermixture of clay and is thin and highly porous. It follows that in a dry season the crops are short and meagre. The Scots fir, however, is partial to such a soil, and its ready growth helps with the aid of the natural birches to embellish the Deeside landscape.
In the Cairngorms brown and yellow varieties of quartz called “cairngorms” are found either embedded in cavities of the granite or in thedetritusthat accumulates from the decomposition of exposed rocks. The stones, which are really crystals, are much prized for jewellery, and are of various colours, pale yellow (citrine), brown or smoky, and black and almost opaque. When well cut and set in silver, either as brooches or as an adornment to the handles of dirks, they have a brilliant effect. Time was when they were systematically dug and searched for, and certain persons made a living by their finds on the hill-sides; but now they are more rare and come upon only by accident.