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Tuesday, 12 February 2008

Best Scottish Fossils

The Queen Elizabeth Forest Park Visitor Centre near Aberfoyle, in The Trossachs, has an illustrated display of local geology, and an illustrated leaflet describing a sign-posted Fault Trail through the forest to examine the remains of this colossal activity in its much more tranquil modern setting.

From the hill above Lime Craig Quarry there is a spectacular panoramic view of the Trossachs, and beneath your feet an impressive view of an entire ocean. This ocean is long-dead, but evidence for its existence comes from fossils that have been discovered in small outcrops of rocks in Lime Craig Quarry and the surrounding areas. The information is far from complete, but the piecing together of the sparse fossil evidence would indicate that this ocean developed around 500 million years ago and was in existence for at least 40 million years. Yet at Lime Craig Quarry today, the remnants of this ocean are found in a belt no more than 135yds wide.
The fossils in the limestone at Lime Craig are not museum specimens. This is because they are so small that they have to be looked at under a microscope, and also because a natural process in the rock causes the original fossil shell to be replaced by silica, which forms a replica shell, so that these fossils are not particularly attractive. The rock in which they are found is grubby and highly altered, and is now often buried beneath considerable thicknesses of boulder clay. Yet over 30 different types of Ordovician fossils, formed in the second period of the Palaeozoic era, have been recovered from this limestone, and some of these have proved to be identical to those that occur in rocks of similar age in America. All are marine creatures, such as brachiopods, trilobites and snails. A few yards to the north of the limestone, much younger, and even smaller, microfossils have been found in shallow-water sandstone, providing an upper-age estimate for the ocean. During excavations of the quarry other rock types were found, including black shales, which are
characteristic deposits of deep marine environments, and also indications of volcanic activity.

The geological importance of these fossils stems from their situation along the Highland Boundary Fault, which separates the Highlands of Scotland from the Lowlands. The Highlands are composed of very old, metamorphosed Dalradian rocks, over 570 million years old, while the Lowlands of Scotland are made up of much younger Devonian rocks, about 400 million years old. Standing in Lime Craig Quarry, the hills to the south are Devonian and those to the north are Dalradian; in between lies the Highland Boundary Fault and the slivers of rock such as in Lime Craig Quarry, which are known as the Highland Border Complex. Prior to the discovery of the fossils the age of the Highland Border Complex was unknown, and this greatly hindered attempts to reconstruct the geological history of the area.

Since their discovery, however, it is clear that this narrow zone contains the remains of an Ordovician ocean, which has been destroyed by movements along the Highland Boundary Fault. It is now accepted that the earth's crust is made up of a number of plates that move around, and herein lies the answer to an ocean 135yds wide; in Lime Craig Quarry there is evidence for the lateral movement of two plates that crushed between them an entire ocean, or possibly, even, more than one ocean.

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