050 James Geikie — On Changes of Climate. 



liave not been crumpled only ; they are everywhere cut through by 

 the overlying Boulder-clay, and large portions have been carried 

 away. Indeed, when we compare the bulk of these beds with that 

 of the Till, we must at once allow that they form but a small frac- 

 tion of the glacial deposits. But the geological importance of a 

 deposit is not usually measured by its bulk. In exposed positions, 

 such as hill-tops and hill-slopes, the Boulder-clay never contains 

 intercalated beds, nor do these often occur save as interrupted and 

 fragmentary patches in places which ajopear to have been open to the 

 full sweep of the ice-currents. 



It is in the numerous small tributary valleys, whose trend is often 

 at right angles to the path, followed by the ice, that the intercalated 

 beds of the Boulder-clay attain their best development. Nor is it 

 difficult to see why this should be so. The ice-sheet which, as we 

 know, flowed along the principal valleys, must frequently have 

 crossed the lateral and subsidiary valleys at an angle. In the main 

 valleys the glacier mass would exert its full influence, but it would 

 not be able to do so in the narrow lateral valleys and ravines. The 

 ice and Boulder-clay would merely topple into the glens referred to, 

 and gradually choke them up, and the main mass of the glacier would 

 then pass on over the whole. There could be little or no glacial 

 erosion in the beds of these valleys, consequently any superficial de- 

 posits they might contain would be preserved. And such in point of 

 fact is the case. Many of the lateral streams which feed the principal 

 rivers are found to flow in deep rocky channels, which have either 

 been partially or entirely eroded since the close of the Grlacial epoch. 

 And, curiously enough, these newer river-cuts frequently intersect 

 the pre-Glacial and inter-Glacial ravines which are seen to be 

 entirely filled up with successions of Boulder-clay and sand, silt, 

 and gravel. In the mining operations of our coal-fields, not a few 

 obliterated water-courses have been traced out, often for long dis- 

 tances, in the underground workings, the coal-seams being mined 

 up to the edge of the buried ravines. There is often nothing at the 

 surface of the ground to indicate that a buried valley lies under- 

 neath. A wide pall of Boulder-clay usually stretches across the 

 whole district, and were it not for stream cuttings and mining opera- 

 tions, we should certainly never have guessed the existence below of 

 old deserted water-courses, over the sites of which whole hills of 

 Boulder-clay are sometimes piled up. 



It can easily be seen that these ravines have not been filled up all 

 at once. The beds of sand and silt which they contain are frequently 

 finely stratified, and layers of gravel and coarse shingle often alter- 

 nate with the finer materials. Interstratified with these deposits 

 come masses of Boulder-clay, the stones in which are well polished 

 and scratched. By successive accumulations of all these beds, the 

 old ravines and valleys were slowly and gradually levelled up and 

 obliterated. There were thus distinct pauses in the formation and 

 deposition of the Till. 



The intercalated beds consist, as I have just said, of silt, clay, 

 sand, and gravel — sometimes the coarser, and at other times the finer 



