Yol. 68.] &tACIATIOi^ OF TfiE BLACK COMBi: DISTEICT. 445 



X. The Up pee Botjlbee Clay. 



The origin of the Upper Boulder Clay, which frequently forms 

 the highest division of the drifts, presents a problem to which 

 this district, so far as I have studied it, furnishes few definite 

 clues. 



It is usually red, of a more loamy texture than the Lower 

 Clay, and often contains very few stones ; yet at other times 

 it is full of stones, and the Lower Clay, becoming loamy, resembles 

 it in tint and texture, making it diflScult, were the gravels absent, 

 to separate the two. 



If — as is highly probable — the Upper Clay is the result of a 

 temporary re-advance of the ice, which had withdrawn itself for 

 some distance from the present coast-line, it is just the type of clay 

 that we might expect to find deposited on the sands and gravels : 

 for it would be naturally composed chiefly of the upper part of the 

 previously-formed Lower Clay, mixed with a certain amount of 

 sand and gravel, incorporated when it mounted and overrode those 

 deposits. A re-advance would imply a lowering of temperature 

 and a possible hardening of the gravels by freezing. The ice would 

 then easily override the latter, and deposit the material which it 

 dragged up with it on the surface of the gravels as loamy Upper 

 Boulder Clay. 



If we require a precedent for the overriding of gravels by ice 

 and the deposition of till and morainic matter upon them without 

 destroying them, it is furnished by examples in JNunatak Piord, 

 Hidden Glacier Valley, and Eussell Piord in Alaska.^ The surfaces 

 of these gravels (up to 150 feet in thickness) are carved into a 

 series of swinging, undulating, or dome-shaped curves, which 

 terminate the layers of gravel, and are characteristic of ice-erosion. 

 A layer of till, or hummocks and ridges of morainic material 

 deposited during the period of overriding, covers much of the 

 surface-area. 



If the Upper Boulder Clay of the Black Combe district was 

 formed in this manner, the advance of ice did not greatly disturb 

 the sands and gravels, and seems to have spent itself before the 

 base of the hills was reached. 



XI. Coeeie-Glaciees. 



After the maximum giaciation, when the ice had abandoned the 

 hill-tops for the low ground, small glaciers lingered in some of the 

 upland valleys, occupying the sites of those which must have been 

 initiated there in the early stages of giaciation. They may have again 

 come into existence during the formation of the Upper Boulder Clay. 

 Black Combe is a typical mountain-corrie, the floor of which hangs 

 above that of Whitecombe. It was occupied by a corrie-glacier, 



^ R. S. Tarr, 'The Yakutat Bay Eegion, Alaska' U.S. Geol. Surv. Prof. 

 Paper No. 64 (1909) pp. 126, 128, & 132. 



