Notes on a transported mass of igneous rock at Comiston Sandpit, near Edinburgh

Abstract

THE glacial sands of Comiston near Edinburgh have long been used for building purposes, and were also of use at one time in furnishing the city with part of its supply of water. Before the sand-pit, which is the subject of these notes, was opened, the supply of sand for building was obtained from an extensive pit on the east side of the public road from Edinburgh to Fairmilehead. When the supply became exhausted, the present one was opened on the west side of the road, and opposite to the old pit. The quarrying of the sand at first was carried on eastwards towards the road. Here rock, most probably in situ, began to appear coming up through the sand, and the quarrying was stopped in this direction and diverted westwards. About three years ago the workings exposed a sheet or mass of rock of volcanic origin lying horizontally on the top of the current-bedded sand and gravel. It varied from 4 to 8 or 9 ft. in thickness, and a length of 45 or 50 yds. in an east and west direction was at one time exposed. The appearance at the present time is somewhat different.The diagram (Fig. 1) shows the contours of the ground near Comiston, and also indicates roughly the extent of the sands and gravels. It will also serve to show the probable method of their formation. We must suppose that a lake was dammed on one side by the margin of the ice-sheet which occupied the central valley of Scotland, at a period of its retreat when it extended only a little to the south of where the city of Edinburgh now •stands. The southern boundary of the lake must have been formed by the Fairmilehead ridge, and the eastern by the Braid Hills. In this lake the deposits were laid down. Confirmatory evidence of its existence may be found in the presence, somewhat west from Fairmilehead, of an old overflow channel, shown in the diagram, and draining to the south across the ridge. The depth of sand in the eastern part of Comiston sand-pit is only about 20 ft. ; it rests on a floor of boulder-clay, perhaps extremely thin, as at one or two points rock was found projecting through it which may have been in place. In the western part of the quarry the sand is much mixed with gravel, and the total thickness is greater. The gravel consists of rounded pebbles whifch are mainly Carboniferous. A few, however, are of High- 21, 2015 at Monash University on June http://trned.lyellcollection.org/ Downloaded from