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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 CITP CEng FGS FRAS (Yes, the webmaster is also a geologist)

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