The Loch of Strathbeg, which lies on the east coast not far from Rattray Head, is a brackish loch of some interest. Two hundred years ago, we are told, it was in direct communication with the sea and small vessels were able to enter it. In a single night a furious easterly gale blew away a sand-hill between the Castle-hill of Rattray and the sea, with the result that the wind-driven sand formed a sand-bar where formerly there was a clear
Loch Callater, Braemar
water-way. Since that day the loch has been land-locked and though still slightly brackish may be regarded as an inland loch.
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 are igneous rocks. 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
To these may be added a third kind of rock called metamorphic, 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.