408 PREHISTORIC EUROPE. 



many large boulders, confusedly arranged for the most part, and 

 betokening the action of more or less torrential water. At the 

 time those gravel-banks were being formed the streams must 

 not only have been deeper but broader than at present, seeing 

 that the modern alluvial flats consist generally of more orderly 

 arranged materials, which are flanked on one or both sides by 

 the truncated ends of the older deposits. In short, we have 

 here the very common phenomenon of " valley within valley." 

 The accompanying diagrammatic section will show more clearly 

 what is meant. Eesting upon the rocky bottom of the valley is 



Fig. 12. — Diagrammatic Section of Postglacial and Recent River-gravels, etc. 

 t, till ; g, glacial gravels ; 1, 2, 3, successive terraces of alluvial origin. 



a layer of till, t, with overlying glacial gravels, g, the presence 

 of which proves that the valley was in existence before the 

 Glacial Period. Upon the denuded surface of the till and 

 glacial gravel comes a succession of terraces, all of which have 

 been formed in Postglacial and Recent times. The highest of 

 these (1) is of course the oldest of the series — it may be 10 or 

 15 feet, or even more, above the level of the present stream. 

 It points to a time when the river was able to flood the whole 

 bottom of the valley from side to side. At a lower level a 

 second terrace (2) appears. As this terrace occurs within the 

 former one, it proves that the river eventually became reduced 

 in volume, and, no longer able to flood the whole valley, pro- 

 ceeded to cut for itself a channel through its own deposits. The 

 third and lowest terrace represents the surface which the stream 

 in our own day covers during flood. Its diminished width and 

 lower level indicate a further reduction in the volume of water. 

 I do not of course mean to say that all those alluvial terraces, 

 which are now just beyond the reach of rivers in flood, were 

 formed at a time when those rivers were deeper and broader. 



