W-E trending striae (above) and grooves (below) on the upper western slopes
"The object of this excursion was to visit the remarkable series of grooved and scratched rocks which had been discovered on the western edge of the cliff-like boundary of Corstorphine Hill. The glacial origin of these groovings on the rocks was then occupying the attention of geologists. Robert Chambers.... was now enabled to show us these groovings and scratchings on the rocks near Edinburgh. In order to render the records more accessible, he had the heather and mossy turf carefully removed-- especially from some of the most distinct evidences of glacial rock-grooving. Thus no time was lost, and we immediately saw the unquestionable markings. Such visits as these are a thousand times more instructive and interesting than long papers read at scientific meetings. They afford the best opportunity for interchange of ideas, and directly produce an emphatic result; for one cannot cavil about what he has seen with his eyes and felt with his hands."
James Nasmyth: Engineer, An Autobiography 1897
The grooves and scratches on the rocks at Corstorphine Hill were recognised first by Sir James Hall in 1812. He interpreted the streamlining as the result of the biblical flood.
Abraded slabs on the west side of the hill
Significance: a quartz-dolerite ridge with fine examples of roches moutonnées that was an important locality in establishing that ice sheets covered lowland Scotland.
Corstorphine Hill provides one of the most accessible locations in central Scotland to examine the effects of glacial erosion on a band of hard rock. Here a thick quartz dolerite sill provides the tough, jointed rock in which roches moutonnées are typically formed. The sill dips gently to the west and so forms a cuesta, with the scarp slope facing east. The ridge lay across the line of flow of the last and earlier ice sheets and so has been re-shaped by glacial erosion.
The Google Earth image above shows that the ridge is strongly asymmetric. The dip slope of the dolerite has been stripped of overlying sandstone and exposed to glacial abrasion. The down-ice or scrap face has been steepened by quarrying of blocks beneath the ice to give a series of crags.
A small roche moutonnée, showing abraded (up-ice) and plucked (down-ice) faces
On the ridge the effects of ice moulding are widely apparent. A large number of roches moutonnées are developed towards the ridge crest. These landforms show a wide range of shapes and sizes and the orientation and density of joints is the dominant control on form. The ridge is cross cut by a series of ravines. Exposure is poor here but it is highly likely that these depressions lie along master joints or in zones of high joint density.
One of the many wooded ravines that cut though the ridge crest
Superimposed on the abraded surfaces on the up-ice side of the ridge are small roches moutonnées, grooves and striae. These microforms are oriented west-east and provide a record of the passage of the last ice sheet.
Plucked lee-side cliff on the sides of a ravine
The hierarchy of features is testament to the processes operating beneath the glaciers. Ice forced to move uphill will exert great pressure of rocks embedded in its base. This debris will abrade, scratch and groove the rock surface. Towards the crest of the ridge the basal pressures will be reduced, allowing ice to flow around and mould rock bumps. Cavities may open behind these bumps if the ice flow is fast or the ice is thin. The movement of meltwater towards these cavities, together with stress release in rocks within them, would encourage block quarrying and the plucking of loosened blocks from the lee sides of roches moutonnées.