144 



here. Along those directions the rock surface is ridged and 

 furrowed most, and the form of these ridges and furrows has been, 

 so to speak, propagated, through the drift afterwards moulded over 

 the surface as the ice melted. 



It will also help us to understand the other cases wherein the 

 axes of the drumlins do not lie in the same direction as that of the 

 glacial striae. The direction of the striae merely represents the 

 directions of the very last movement of the ice, which direction 

 may have prevailed only for a few thousand years ; while the 

 direction of the greater ridges marks the prevailing line of move- 

 ment through a period very much longer. 



Formation of the Gntta Percha Clays. — The origin of the finely- 

 laminated beds of clay known as gutta percha clays, which are so 

 commonly found in association with the more clayey forms of drift, 

 meets with a simple explanation on the view above set forth. They 

 are due to the successive deposition of films of clay deposited from 

 trickling sheets of muddy water flowing from the ice over the clay 

 surface beneath. Their formation necessarily occupied much time; 

 for it seems clear that each of the layers, many of them as thin as 



Fig. I. 



Laminated Clays and Loams move or less affected by thrust planes due to 



the movements of the melting ice above it. Length about 12 inches. 



Cutting in the Midland Railway, near Langanby. 



