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Chapter 6. Geology

From the point of view of origin, all rocks belong to one or other of two groups. There are the igneous rocks, which have been at one time in a molten condition, and which have become consolidated by a process of crystallisation ; while the derivative rocks, directly or indirectly, result from the decay of pre-existing rocks. Familiar examples of igneous rocks are the lavas from modem volcanoes. Sometimes, however, the molten matter fails to reach the surface, and is consolidated, as granite for example, in or between other rocks. It is then called intrusive. Derivative rocks are often spoken of as sedimentary, because for the most part they have been deposited as sediments in the flow of lake or sea. They may be recognised in the field by their bedded or stratified character. Igneous rocks, on the other hand, are unbedded. Many rock masses have been so profoundly altered by heat, by pressure, and by other causes, that their original characters are more or less obscured. Such rocks are termed metamorphic. Examples of these are the widespread mica schists and gneisses.

The deposits now forming in the sea floor tend to be arranged in approximately horizontal layers. Very often, however, as a-result of coastal movements the sedimentary rocks have been tilted (sometimes, as at Stonehaven, the bedding planes are quite vertical) ; or again they have yielded to pressure by folding or fracturing. The folding may be simple, as in the rocks which underlie the Howe of the Mearns; or complicated, as in the schists of the Grampians. A splendid illustration of a fracture or fault on a big scale is seen in the “ Highland Fault,” which forms the geographical boundary between the Highlands and the Midland Valley of Scotland. It enters Kincardineshire at the Woods of the Burn, and reaches the North Sea at Garron Point, near Stonehaven.

To many the chief interest of geology lies in the study of fossils, the remains of plants and animals preserved in the sedimentary rocks. Fossils enable us to ascertain the relative age of rocks and to classify them in groups and systems. The oldest rocks of the earth’s crust, the Pre-Cambrian, contain few fossils. Overlying these are four great groups, which, taken in order of age, have been named as follows: (1) Primary or Palcsozoic; (2) Secondary or Mesozoic; (3) Tertiary or Cainozoic; (4) Post-Tertiary. The rocks of known age in Kincardineshire belong either to the Primary group or to the Post-Tertiary. The Post-Tertiary deposits include the boulder clays and fluvio-glacial gravels and sands, the raised beaches which fringe the coast, the alluvial terraces or haughs of the river valleys, and the peat mosses.

Considering first the solid rocks of the county, we find they are of markedly different character on opposite sides of the Highland Fault. To the north of that great fracture they belong mainly to the Dalradian series, to the south to the Old Red Sandstone. Between the Dalradian rocks and the Highland Fault, however, at the Woods of the Burn, at Glensaugh, at the Bervie Water, and at Elfhill, areas occur to which has been applied the term Highland Border rocks. On the coast between Cowie and Garron Point, but on the south side of the Highland Fault, rocks similar in their lithological characters have yielded fossils which indicate that they are in all probability of Cambrian age. Another interesting suite of rocks occupying the coast section from Ruthery Head to Stonehaven Harbour, and extending inland for 7 miles, has recently been shown to contain characteristic Silurian fossils.

The Old Red Sandstone system of Scotland is subdivided into Lower, Middle, and Upper. Rocks belonging to the Lower series occupy most of the southern half of Kincardineshire. The Middle series is absent, and the Upper is found only in a narrow tract along the coast near St Cyrus.

The Dalradian rocks may be studied most conveniently in the cliffs between Garron Point and the Bay of Nigg, but numerous good sections are exposed in the streams which traverse the hills between the valley of the Dee, and the border of the Highlands. Intrusive rocks of various types are found associated with the Dalradian rocks. In the neighbourhood of Banchory, for example, these have been “flooded” with a very old granite ; and later dykes are everywhere abundant. Further, the dominating features in the scenery of the northern half of the county are produced by intrusive rocks— the “ newer ” granites on either side of the valley of the Dee.

The Highland Border rocks consist of two groups : an older series (probably Cambrian) made up of green pillowy lavas, associated with red jaspers, green cherts, and black shales ; and a younger series of conglomerate grits, limestone, and shales. Both groups show a splendid development at the “Rocks of Solitude” in Glenesk ; and the fossiliferous shales of the older series may be hammered in the cliffs at Craigeven Bay, Stonehaven. The most abundant fossils are early types of Brachiopods or lamp shells. The limestone of the younger series was at one time extensively worked.

During early Silurian times the region to the north of the Highland Fault began to undergo compression and elevation. The Dalradian rocks and the rocks of the Highland Border series were thrown- into great folds; the coastal movements moreover heralded a violent outburst of volcanic activity. We may picture the Grampians of that period as a lofty mountain range with numerous active volcanoes, snow-covered doubtless, and resembling perhaps the Andes of the present day.

The magnificent cliffs from Stonehaven southwards afford splendid opportunities for the study of the Lower Old Red Sandstone. Coarse conglomerates predominate, but occasionally give place to micaceous sandstones, while at intervals the succession of bedded rocks is broken by massive piles of lavas. Some of the bedded rocks, too, on close examination, prove to be volcanic tuffs, the consolidated “ ashes ” of the contemporaneous volcanoes. Tuffs occur also at Cowie, where their presence shows that volcanic activity had already begun in Silurian times. It continued until almost the close of the Lower Old Red Sandstone period. The hard resistant lavas form most of the high ground in the southern half of the county. The Garvock Hills, for example, are built up for the most part of a great succession of lava flows, and show beautifully from certain points of view the characteristic step-like arrangement which suggested the old name of “trap” rocks.

At the close of Lower Old Red Sandstone times coastal movements again made themselves felt in no uncertain fashion. The rocks of this period were compressed into simple “saddle-shaped” and “trough-shaped” folds—the Howe of the Mearns marks the position of one of the latter—and "then, too, in all probability, was initiated differential movement along the line of the Highland Fault. The forces of denudation became active, and from the disintegration of the Lower Old Red Sandstone and older rocks were built up the bedded rocks of the Upper Old Red Sandstone. The latter formation occurs in the coastal track between St Cyrus and the mouth of the North Esk, and is everywhere separated from the Lower Old Red Sandstone by lines of faulting. A vast epoch of time intervened between the deposition of the two formations. No fossils have been obtained so far from the Upper series in Kincardineshire, and the age of the rocks is inferred from their structural relations and from their lithological resemblances to fossiliferous rocks of like age in other parts of Scotland. One of the most characteristic rocks is a variety of nodular limestone known as “cornstone.” This, like the limestones of the Highland Border, was at one time burned for lime.

Now follows, as regards our county, a great gap in the geological record. Of the story of the remainder of the Paleozoic epoch, and of the whole of the Mesozoic and Cainozoic times the rocks of Kincardineshire tell us but little, and that little very indirectly. In the Upper Old Red .Sandstone period the highest forms of life were primitive fishes. Amphibians, reptiles, birds and mammals had, in succession, been evolved.

The Post-Tertiary deposits in Kincardineshire consist mostly of accumulations of sand and gravel, and of boulder clay or till with its characteristic striated boulders. They tell us of a time not so very long ago, geologically speaking, when the whole of Scotland, with the exception of a few of the highest mountain peaks, >vas buried deep in the ice sheet of the Great Ice Age.

The striated stones are the tools with which the ice sheet accomplished its work. How effectively that work was done is evidenced by the rounded, flowing contours of our hills, by the presence of boulder clay and erratic blocks, by the glacial grooving on a big scale wherever belts of soft rock lay in the path of the ice, and by the preservation of the ancient bottom moraine, the great thickness of till which conceals the solid rocks over much of the county. That the minor surface features are largely glacial in origin cannot for a moment be doubted. One instance must suffice. No one travelling along the Howe of the Mearns can fail to note the contrast offered by the bordering hills. On the one side, the even boulder-clay-covered slopes of the Garvock Hills rise gently from the plain ; on the other, every valley opening from the Grampians is fronted by one or more steep-faced terraces. The terraces consist of sand and gravel deposited in lakes formed at a time when, while the local hills were free from ice, a great lobe of the Highland ice sheet still occupied the Howe. Similar phenomena are seen in the wide valley of the Dee.


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