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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