stirling-1963-vol-1/05_036

Transcription

INTRODUCTION : GENERAL
headwater tributaries descend from the north-eastern flank of the Ben Lomond massif, while
the south-western slopes fall directly to the shore of Loch Lomond. Part of the region south
of Falkirk drains northwards towards the mouth of the Carron and the remainder eastwards
to the Avon.
The north-western portion of the county, comprising Ben Lomond and its associated
heights, lies north-west of the Highland Boundary Fault, which here corresponds with a line
drawn from Aberfoyle to Inch Murrin. ¹ The rocks north-west of the Fault are predominantly
metamorphosed sediments, such as various types of schists and schistose grits, quartzites,
slates etc., together with altered igneous rocks, such as epidiorite. The rocks have been
invaded, at a time subsequent to the dynamic movements that folded and metamorphosed
them, by intrusions in the form of bosses and dykes comprising diorite, felsite, porphyrite,
lamprophyre, dolerite, and so on. The metamorphic rocks are known as Dalradian, and the
concensus of geological opinion seems to be that they are of pre-Cambrian age. Along the
south-eastern side of the Fault there are found masses and lenticles of little-altered shale,
chert and limestone, associated in places with lava and serpentine. These are called the High-
land Boundary Rocks, and fossils show that they are either Cambrian or Ordovician in age.
South-east of the line of the Fault the county is floored by a belt of Lower Old Red Sandstone
rocks, some eight miles in width and trending from north-east to south-west, which extends
to the Lennox Hills and flanks their northern side. This is succeeded, on their lower slopes
by a belt of Upper Old Red Sandstone about two miles in width, which is in turn overlain by
Lower Carboniferous rocks known as the Cementstone series; these latter outcrop at the base
of the slopes of the Lennox and Kilpatrick Hills, which are themselves built up of lava flows
with necks and intrusions, all of Lower Carboniferous age. South and east of these basaltic
hills the Carboniferous Limestone group floors a large area, being overlain in places by
Millstone Grit and Coal Measures; but the continuity of the Carboniferous rocks is broken
for some miles southwards from Stirling by a massive intrusion of quartz dolerite, of which
Stirling Castle Rock and Sauchie Craigs form the most striking features. The lowest-lying
lands along the Forth - a narrow strip beside the river upstream from Stirling and a much
wider area fronting on the Firth from Cambuskenneth to the mouth of the Avon - are covered
by alluvial clay; while the parish of Logie, across the Forth from Stirling, is divided between
this alluvium and Old Red Sandstone lavas. Boulder Clay is widely spread, and ranges from a
stiff puddly clay at one end of the scale to a lighter sandy one at the other. The latter provides
a much more fertile soil. Although much material has been carried into the county by ice-
sheets travelling from the Grampians, the clay is mainly derived from local rocks and reflects
their qualities in respect of agricultural fertility. Raised beaches ² can be seen in many places,
and during the Atlantic climatic phase the sea must have reached a point sixteen miles above
Stirling. ³ This same transgression must have flooded much of the carse land south-east of
Stirling and between Airth and the mouth of the Avon.

1 For the geological data contained in this paragraph the Commissioners are indebted to Mr. R. Eckford. F.S.A.Scot., late of
the Geological Survey of Great Britain.
2 Described by Dinham, "The Geology of the District around Edinburgh: Stirling District", in the Proceedings of the
Geologists' Association, xxxviii (1927), 486 ff.
3 The levels of this beach decrease gradually from about 49 ft. at the head of the valley to 43 ft. near Stirling, and to 25 ft. or
less near Grangemouth (ibid.). See also p. 18 (infra.).

-- 2