L32 I. C. RUSSELL CJRFACE GEOLOGY OF ALASKA. 



It n | » J < • • : i r - {rem the formula and the tnt»lo that the <!<■) >t h< to which any specified 

 full of temp tes vary inversely as the square runt- of the corresponding 



tim 



To And liow l')iiL, r a time is required to produce a given fall in temperature at a 



given depth, we must find / from the pr ding equation whe"n all the other factors 



are known. Thus, Buppose that, under the conditions assumed in the above example, 



we require the time when the temperature will full to 30° at a depth of 200 feet, the 



equation becomi 



200 



40 , i 



30 . 2 r — • 



— - = 1 — — : — : I e dz 



50 





This | 180 j 



The conclusion reached by Mr. Woodward indicates that the freezing of 

 eveu the deepest ice-stratum reported in the Arctic- might have resulted 

 directly from a mean annual temperature no lower than now prevails in 

 northern Alaska. The conductivity of the frozen soils and subsoils of 

 Alaska lias not been investigated, but is probably less rapid than in the 

 strata in which the value determined by Thomson and Tail was obtained. 

 Other values may be substituted in the formula, out any probable variations 

 from those used would not affeel the general conclusion reached. 



Although tin- passage of hrnt through the surface layers in Arctic regions 

 i- bIow, yel it is apparent that the length of time since a mild climate existed 

 there is sufficient, even under existing conditions, to allow of the freezing of 

 strata Beveral hundred feet below the surface. The mean annual temperature 

 pfthe nonglaciated portion of Alaska during the glacial epoch must have 

 been lower than at present — at least such 1 am confident would be the con- 

 clusion of the majority of geologists, — and their Beems good reason for believ- 

 ing that the freezing of the tundra began In Pleistocene time and continued 

 to the present day. An increase in the thickness of the frozen layer, owing 

 to the influence of a mean annual temperature below 325 F.and the deposi- 

 tion of a succession of frozen layers, as suggested elsewhere, may have com- 

 bined to produce the results now observed. 



THE FROZEN MOSS-LAYER IS \ GEOLOGICAL IGENT. 



Throughout Alaska drainage is obstructed by the universal mossy cover- 

 ing. There is an absence of -mall Btreams; rills and even creeks of con- 



rable size arc frequently ponded and transformed into swamps by the 

 progressive growth of vegetation from their banks. Not only are the denud- 

 ing effects of rain-drops falling on the land entirely counteracted by the 

 most areas, but the water is retained by the sponj 



mo-- and allowed to Beep -lowly away. The Btreams formed by the water 



r tillering through the moss are clear and limpid, and consequently unable 



