56 James Geikie — On the Age of the Crofthead Deposits. 



cleared away. It is only the tailing-off of tlie Boulder-clay which, 

 as we follow it towards Shillford, creeps a little further up the 

 southern slopes of the valley, but is there "very thin.^ No overlying 

 Boulder-clay, however, occurs on this side of the valley as we 

 approach the spot from which section Fig. 1 is taken. The slopes of 

 the rock surface below the drift deposits, as shown in the two 

 sections, are protracted from the angle formed by the sides of the 

 valley, and the depth from the level of the railway to the rock 

 thus obtained agrees closely with that ascertained from borings. If 

 the reader refers to my former communication (Vol. V., p. 393) he will 

 there find stated the evidence for the existence of the Boulder-clay 

 underlying the lacustrine deposits, which is represented in the 

 accompanying woodcuts (Figs. 1 and 2) at &1. 



Quite recently the railway operations were interrupted by a 

 landslip. The lacustrine deposits had been so deeply cut into that 

 the downward pressure of the superincumbent Boulder-clay forced 

 the soft wet beds below to bulge forward, and I was told that the 

 rail-road was squeezed up some six or seven feet. The navvies had 

 only repeated what the stream had done before. It appears clear to 

 me that Boulder-clay, resting on lacustrine deposits, once filled up 

 the entire hollow from B to D. By and by the stream cut its way 

 down through the Till and cleared it away from c to D. The under- 

 lying soft, wet silt, sand, and clay, now pressed down unequally by 

 the Till (&2), yielded, and bulged up at C, where, of course, the 

 stream gradually washed it away. Thus the Boulder-clay, I believe, 

 is now at a slightly lower level than formerly. Similar landslips 

 may often be witnessed along the river-courses in the west of Scotland. 

 In the valley of the Calder Water (Eiver Avon) for instance, at 

 Calderbraehead, the river has cut its way down through a broad and 

 gently sloping terrace of Boulder-clay into deposits of soft clay, silt, 

 and sand, which, from borings, have been ascertained to be of great 

 thickness. Now, just as in the Cowden valley, so here, the weight 

 of the overlying Boulder-clay has forced the subjacent denuded and 

 exposed deposits outward into the stream, which is rapidly cariying 

 them away, and the result is that the surface of the ground adjoining 

 the river is permanently lowered. Thus, neither in the case of the 

 Cowden Valley, nor in that just mentioned, have the landslips been 

 caused by the sliding forward of the Boulder-clay upon the stratified 

 deposits, but by the undermining, displacement, and actual abstrac- 

 tion of the soft foundations on which the Boulder-clay rests. 



I saw the section at Crofthead, in November last, in company 

 with my friend and colleague Mr. Croll, and I shall only add that 

 he was as much surprised as myself that any one could fail to see the 

 truly intercalated character of the lacustrine deposits which have 

 yielded the mammalian remains. 



^ If, therefore, I had taken the section (Fig. 2) across the valley, a little nearer 

 Shillford, it would have shown a larger portion of Boulder-clay in the position re- 

 presented in the aunexed woodcut at 42*. 



