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The Lowlands of Mid-Scotland, or the Central Plain, constitute a broad
depression with south-westerly to north-easterly trend lying between
the Highland line that runs from the head of the Firth of Clyde to Stonehaven
and the pastoral uplands that stretch from Girvan to Dunbar. They may
be regarded as a long trough of younger rocks let down by parallel dislocations
between the older masses to the south and north. The lowest of these
younger rocks are the various sedimentary and volcanic members of the
Old Red Sandstone. These are covered by the successive formations of
the Carboniferous system. The total thickness of both these groups of
rock cannot be less than 30,000 ft,and, as most of them bear evidence
of having been deposited in shallow water, they,could only have been
accumulated during a prolonged period of depression. The question arises
whether this depression affected only the area of the midland valley,
or extended also to the regions to the north and south; and so far as
the evidence goes there is ground for the inference that, while the
depression had its maximum along the line of the lowlands, it also involved
some portion at least of the high grounds on either side. In other words,
the Old Red Sandstone and Carboniferous rocks, though chiefly accumulated
in the broad lowland valley, crept also over some part of the hills
on either side, where a few outliers tell of their former extension.
The central Lowlands are thus of great geological antiquity. During
and since the deposition of the rocks that underlie them the tract has
been the scene of reneated terrestrial disturbances. Long dislocations
have sharply defined its northern and southern margins. By other fractures
and unequal movements of upheaval or depression portions of the older
rocks have been brought up within the bounds of the younger, and areas
of the younger have been enclosed by the older. On the whole, these
disturbances have followed the prevalent north-easterly trend, and hence
a general tendency may be observed among the main ridges and valleys
to run in that direction. The chains of the Ochil, Sidlaw, Pentland,
Renfrew, Campsie and Fintry Hills, and the valleys of the Strathmore,
Firth of Tay, and the basin of Midlothian may be cited as examples.
But the dominant cause in the determination of the topographical prominences
and depressions of the district has been the relative hardness and softness
of the rocks. Almost all the eminences in the Lowlands consist of hard
igneous rocks, forming not only chains of hills such as those just mentioned
and others in Ayrshire and Lanarkshire, but isolated crags and hills
like those on which stand the castles of Edinburgh and Stirling, and
others conspicuous in the scenery of Fife and the Lothians. Of the three
chief valleys in the central LowPands two, those of the Tay and the
Forth, descend from the Highlands, and one, that of the Clyde, from
the Southern Uplands. Though on the whole transverse, these depressions
furnish another notable example of that independence of geological structure
already referred to.
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