202 Frank Carney 



Since the streams that flow from a glacier usually bear a load, 

 one can understand how, under the first of these conditions, the 

 valleys w^ould be silted up with out wash material. Flood plains 

 thus are quickly developed. If the valley concerned was already 

 in old age, the mass of this material accumulating might locally 

 reduce the grade of the valley floor to the extent of ponding 

 some of its drainage. Minor changes have been thus induced. 



Under the second condition, lakes are always formed in the 

 valleys. The levels of these lakes rise until they find an overflow'. 

 If the overflow channel is in unconsolidated material or in very 

 soft rock, it may be so cut down as to permanently change the 

 course of the drainage. 



In the third condition, the ice at some point moves across the 

 valley or basin of a river, whose general course is parallel to the 

 ice margin. When this happens, the upstream part of the basin 

 is ponded, and, if the ice occupying it rises higher than the lowest 

 point on the basin's rim, the ponded water will escape at that 

 point; but if the ice does not thus completely block the valley, the 

 ponded water up stream wdll flow over the ice or between it and 

 the wall of the valley. In the former case, if this new channel 

 continues in use long enough, it may become the permanent 

 course for the drainage of that part of the original valley. 



In the glaciated area, many drainage changes have been attri- 

 buted to ice. Later investigation of some of these reversals 

 appears to throw doubt on the former explanation. Evidence has 

 been found, showing that the reversals had been accomplished 

 before the glacial period. In another section, I consider some 

 of these cases. 



CAUSE or GLACIAL PERIOD 



Many explanations have been offered for the extensive glacia- 

 tion of certain parts of the continents. Most theorists have pro- 

 ceeded from observations made in our temperate regions. Above 

 the snow line in mountains, snow accumulates in accordance with 

 the precipitation. With an increase of snowfall through several 

 succeeding years, the snow line is lowered. Under an equal period 

 of diminished precipitation, the altitude of the snow line rises. 

 It is urged,* therefore, that a prevalence through centuries of 

 the former conditions would extend ice fields down the slopes of the 

 mountains and into the adjacent plains. 



