410 PREHISTORIC EUROPE. 



mammals, and fluviatile shells, they have never disclosed a 

 single trace of any marine organism whatever. It is only when 

 we come down to low levels — to heights of 50 feet or so above 

 the sea — that the old river-terraces merge with estuarine flats, 

 in the lower reaches of which sea-shells now and then make 

 their appearance. 



Other writers, again, have maintained that the old river- 

 terraces of certain lowland-valleys, such as those of the Clyde 

 between Nethanfoot and Bothwell, were the beds of ancient 

 lakes. In that district the Clyde flows through a succession of 

 open flat-bottomed spaces, which are connected by comparatively 

 narrow passages. The opener parts of the valley are supposed 

 to have been occupied by lakes which were subsequently drained 

 by the cutting through of the somewhat contracted outlets 

 which now serve for a course to the river. This, however, has 

 certainly not been the case. The valley as we see it, broad 

 flats and narrower passages alike, is entirely the work of the 

 stream and subaerial agents generally. The river flows partly 

 in a preglacial and partly in a postglacial course. The wider 

 reaches correspond with the former, and the narrower portions 

 with the latter. In the one section of its course it had to deal 

 only with more or less incoherent glacial deposits which were 

 easily demolished ; in the other it had to force a way down 

 through massive strata of sandstone and shale. Hence the width 

 of the valley has been determined by the nature of the materials 

 which had to be removed. Where these were soft and easily 

 undermined and washed away the valley has attained a goodly 

 width ; where they yielded less readily to denudation the river 

 has been forced to content itself with a narrower course. In 

 postglacial times, when the rainfall was greater, the Clyde had 

 little difficulty in flooding the wider reaches of its valley, which 

 during spates would appear as a series of temporary lakes con- 

 nected by broad channels of torrential water. 



If the submarine forests prove that Scotland in early post- 

 glacial times was of larger extent and enjoyed a genial climate, 

 the old estuarine deposits of the Tay, the Forth, the Clyde, and 



