Geology of the Hill 

 

Corstorphine Hill, like many others in the Edinburgh area, is a prominent landmark. Why is it there? Even the most subtle details of the landscape have a geological meaning. Most highs and lows of the topography reflect the underlying materials, usually rocks. The rocks underlying Corstorphine Hill are mostly hidden by a veneer of soil and vegetation but, if you keep your eyes open, you will see places where hard grey rocks come to the surface. Such occurrences are called 'outcrops' and it is from these sparse occurrences that geologists are able to piece together geological maps which depict the distribution of the different rock types on the surface of the land (see the map shown in Figure 1) and permit an interpretation of what happens to them beneath the surface (cross section shown in Figure 1).

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Figure 1

The map and cross section show that the rocks of the area have been folded into a large dome-like structure and Corstorphine Hill and surrounding areas lie on the west side of this structure so that all the rocks are tilted (dip, in geological terminology) towards the west.

 

The rock outcrops on Corstorphine Hill are mainly igneous rock. They bear a variety of marks such as striations or scratches made by the passage of glaciers over the hill, for the area was covered by thick ice during the Pleistocene glaciation. The ice moved up the hill from the west, scraping the surface clean and leaving deep scratch marks as evidence of its former presence. Following disappearance of the ice, outcrops that we can still see today must have been present, for some have been sculpted by human hands - for example, the ca. 5,000 year-old cup marks present near the top of the hill above the Leonardo Hotel.

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The leaflet here is provided by the Lothian and Boarders Regionally Important Geological Site Group, and shows the making of Corstorphine Hill. This leaflet can be purchased via the Edinburgh Geological Society website

Why has this area remained upstanding? How has it withstood the passage of great masses of ice and the ravages of storm and rain over millions of years? It is because it is an intrusion of hard igneous rock. An 'intrusion' is formed when a body of magma forces its way into existing rocks and consolidates into rock itself without ever reaching the surface, in contrast to what happens with lava flows when they come out of volcanoes. A glance at the geological map (Figure 1) shows the Corstorphine Hill area to be underlain by a N-S-trending body of igneous rock, called 'Teschenite and Olivine Dolerite'. This is the geological name for a quite large igneous intrusion that forced its way into much softer sedimentary rocks known as the Lower Oil-Shale Group. These rocks are all thought to be Lower Carboniferous in age - in the vicinity of 350 million years old.

Thus the high ground making up Corstorphine Hill is the result of the presence of a ridge of hard igneous rock that has resisted the effects of erosion by water, wind and ice much more efficiently than the surrounding sedimentary rocks, which are mainly softer fine grained sedimentary rocks such as mudstones or shales (formed from muds), together with some beds of sandstone, such as the Craigleith and Ravelston Sandstones.

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Glacial Evidence

Spectacular ice-smoothed surfaces are a feature of the western slopes of the hill. Grooves and scratches (striae) were cut by boulders and pebbles in the ice, acting like massive sandpaper, as the ice flowed from west to east. These grooves are always oriented at about 80 degrees, as this was the direction of flow in the local ice-sheet.

Boulder clay

The morainic debris of clay, stones, pebbles and boulders left by the ice 15 thousand years ago, seen in upturned trees on the lower slopes. The boulder clay could be cultivated, so it usually underlies the lower grass fields. Both west and east of the hill the boulder clay slopes have ridges oriented at about 80 degrees just as on the glacial pavements.

Geology