On the evidence of the existence of an old river course, previous to the deposition of the boulder clay, at the Water of Leith, above Colinton

Abstract

The condition of the surface of Scotland immediately preceding the deposition of the boulder clay is very little known, nor can much be expected, when we consider the circumstances under which the boulder clay was deposited. That the boulder clay is mostly made up from the rock upon which it rests is evident from an examination of our own neighbourhood. To the north and west of Edinburgh the underlying rocks consist of soft black shales and grey sandstones. Overlying these, in many places we find a great thickness of hard bluish sandy clay, full of boulders of grey sandstone and greenstone; while in the southern district, where reddish sandstones and shales prevail, the boulder clay so closely resembles in colour the rocks upon which it rests that it is often difficult to distinguish the clay from the shales upon which it lies. But a large number of the boulders in the clay offer an exception to this rule, many of them being derived from rocks lying at a considerable distance to the west of where they are found. The boulder clay in this respect presents a curious anomaly to ordinary sedimentary deposits, more especially to those of river or oceanic currents, which generally deposit the larger stones first, and carry the finer sediment farthest; whereas in the case of the boulder clay, the finer sediment has scarcely been removed from the parent rock, while the larger blocks have been transported in many instances to a great distance. The action which