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 we see 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 feet thick. 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 corresponding activity 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 their débris on the surface when the ice disappeared. So much is this a feature of Buchan that one well-known geologist has humorously described it as the riddling heap of creation.