Friends of Corstorphine Hill

Geology

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).

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 Capital Hotel.

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.

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 80degrees just as on the glacial pavements

Walled Garden on Corstorphine Hill - a mini geological trail

There is a story behind the garden wall. It is not merely a bunch of old rocks that were thrown together as a protection from the wind and predatory creatures (including humans). When you look closely at the pieces that constitute the wall, you discover rocks fragments of two kinds, sedimentary and a (very few) igneous. The sedimentary rocks are mostly sandstones but there are also coarser examples (conglomerates) and a few finer ones (mudstones). The sedimentary rocks show various structures ranging from bedding and cross bedding to primary current lineation. Some of the rock fragments contain evidence of ancient life, in the form of plant fragments, both unidentifiable flattened carbon films and ornamented cylindrical shapes representing the stems of ancient plants that were buried and preserved in the sediment when it was laid down. Animals are represented only by trace fossils but they show evidence of the existence of a thriving community of soft bodied animals (worms?) that made their living by ingesting fine sediment (and contained organic particles) and left a tell-tale trace of burrows in the sediment. Next time people get excited about an antique which may be a couple of hundred years old, take them into the garden and (in addition to showing them the modern plants) let them feast their eyes on rocks and fossils that formed over 300 million years ago! The story behind the wall is both very ancient and complex. Much can be learned by looking carefully at these ordinary blocks of stone.

Read more in the The Story Behind the 'Wall', by Grant M Young (1.5MB pdf), a geological photo guide to the rocks incorporated into the rebuilt wall.