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Two major groups and ages
of rocks are found in the Strathearn area of Perthshire. These are separated
by the Highland Boundary Fault (HBF) which is a major crustal fracture
running NE - SW across the map, and across Scotland. To the north of the
fault line lie the crystalline metamorphic rocks of Precambrian to Lower
Cambrian age (older than 590Ma) of the Scottish Highlands. These contain
igneous intrusions of Caledonian age. South of the HBF are younger
sedimentary rocks with lava extrusions (Lower Devonian) which form the Ochil
Hills on the southern fringes of Strathearn. The relative hardnesses of Highland
metamorphic rocks, the sediments and the lavas have resulted in erosion into
the 3 different landscapes of present day Strathearn: the craggy Highlands
rising to 985m, the relatively flat valley floor and the modest Ochil
Hills.
The Dalradian metamorphic
sequence of the Highlands is essentially highly deformed and metamorphosed
marine sediments which were deposited by turbidity currents - slate, schist,
quartzite and marble. The youngest age of Lower Cambrian results from the
finding of trilobite fossils at Callander (just off the SW of the map). The
metaphorphic transition of these rocks took place a result of the Caledonian
Orogeny, a plate tectonic collision active from Mid Ordovician to Mid
Devonian (520-400Ma). The rocks were heated, deformed and fractured in multiple
complex phases resulting is extremely complex field geology. The Comrie granite
pluton is an intrusion dating from the end of the Caledonian Orogeny
(400Ma).
It is worth noting that to
the north of Strathearn the rocks are Moine metamorphics which were heated
and deformed during the Grenvillian Orogeny (1000Ma). Both this and
the Caledonian Orogeny can be seen in N America showing that Perthshire and
N America were part of one continent in ancient times with a major ocean
lying to what is now the south. The Highlands of Scotland were generally
formed at the same time and by the same global crust movements as the Appalachian
Mountains.
By the end of the Caledonian
Orogeny, what is now the Scottish Highlands had been thrown up into a massive
mountain range; this was more like the present day Himalayas than the Scottish
Highlands. The Highland Boundary Fault was active with the area to the south
(the Midland Valley) subsiding relative to the mountains. Huge rivers poured
massive amounts of generally coarse sediment from the mountains into alluvial
fans spreading across the plain to the south. These Lower Devonian rocks
(408-387 Ma) are known as the Lower Old Red Sandstone - though sediments
other than sandstones are present. During this period there was considerable
volcanism across the Midland Valley of Scotland. Thick sequences of basaltic
lavaflows are present. Due to their resistance to erosion relative to the
sediments, these extrusions are left as the persistent high ground that is
the present day Ochil Hills.
A common feature of the Scottish
landscape are vertical dyke intrusions. These are generally
Permo-Carboniferous or Tertiary in age. While the folding and
metamorphism of an Orogeny are manifestations of compression due to crustal
plate collision, dyke swarms result when the crust is stretched. The crust
fractures in vertical planes perpendicular to the stretching forces and molten
rock intrudes along the fractures. The best example seen in Strathearn is
the massive dyke upon which Drummond Castle is constructed.
Several glacial periods have
occurred during the Pleistocene (2Ma-recent) and glaciers occupied
Strathearn at times. The ice eroded steep sided valleys and dug out the
Loch Earn basin. When it eventually melted,
typical glacial features and deposits where left on the landscape..
See also
Earthquake House near Comrie, the Shakin'
Toon.
Jim Murdoch BSc
MSc MBCS CEng FGS FRAS (Yes, the webmaster is also a
geologist) |
