Brief Outline of Pleistocene of Alaska 29 



Porcupine, beds of lignite occur interstratified showing a local 

 drainage or elevation of these basins in the course of their silting up 

 and then further subsidence and deposition. 



As noted by Spurr '" and Collier "' " The silts, though entirely 

 unconsolidated, are in places thrown up into broad, open folds, and 

 at one locality faulting was observed." 



4. snowdrift obstruction of drainage 



None of the facts as now exhibited in Alaska make it necessary 

 to resort to fanciful pictures of the flood plains of the drainage 

 systems being modified or obstructed by great accumulations of 

 winter snows, either to account for the lacustrine nature of the 

 Pleistocene silts themselves or for the deposits of ice found on top 

 of, but never interstratified with, them. Geikie "° refers to Darwin 

 as having suggested " that valleys might eventually become entirely 

 filled with the blown snows of successive years, so as to compel the 

 rivers in summer to rise in flood, and to reach levels which they 

 might otherwise have been unable to attain. That such changes 

 may have taken place again and again is no mere dream." On page 

 665 Geikie concludes: "We are justified, therefore, in the belief 

 that the drainage systems in the low grounds must frequently have 

 been deranged by the presence of such congealed snowdrifts," etc. 



5. pleistocene lake basins 



The Pleistocene lake basins in Alaska were formed by barriers 

 across the general drainage systems as they exist today. The 

 barriers vary in nearly every case. The lakes necessarily did not 

 all become barricaded at the same time. In fact there is reason to 

 believe that in dififerent areas these barriers formed obstructions to 

 the general drainage at varying periods throughout the Pleistocene. 

 Consequently the silt deposits, though similar otherwise, because 

 of common origin, are not all of the same age. Neither were all 

 the Pleistocene lakes drained at the same time, for some of the 

 barriers were more resistant to the erosion that set in with the 

 elevation that closed the period and were not cut down sufficiently 

 until a considerable time after other areas were drained. It is in 

 the last drained areas that the remains of elevated ice are freshest. 



-' Geo!. Yukon Gold Dist.. i8th Ann. Rep. U. S. Geol. Sur.. Part 3. 



"' The Coal Resources of the Yukon, Alaska. Bull. U. S. Geol. Sur., No. 218. 



^' Great Ice Age, p. 663. 



