LAKES HELD IN BY ICE. 471 



fessor Geikie has said, " when controversy ceases the interest in the investi- 

 gation declines." 



Glacial Dams Considered. 



Glacial lakes are of two kinds : those whose waters are retained by 

 morainic barriers ; and others sustained by ice barriers alone. 



The former class is represented in several valleys in the Alps, where lateral 

 glaciers enter and cross greater valleys ; sometimes the glacier carries its 

 lateral moraine across the valley and builds a more or less permanent earth 

 dam. Such lakes remain long after the glacier has melted away, and even 

 when drained show evidence of their origin. A consideration of this class 

 of glacial lakes does not enter into the subject of this paper. 



In Switzerland, Greenland and Alaska other glacial dams are now well 

 known. These are retained by the ice alone. When glaciers, free from 

 morainic materials, descend lateral valleys and cross other valleys, they do 

 not obstruct the rivers, for they continue to flow" beneath the ice. However, 

 there are many places where glacial lakes occur between the ice and the 

 sides of the valleys : especially is this the case where two glaciers meet at the 

 end of a mountain spur, like Lac Tacul in Switzerland. Small glacial lakes, 

 like the Marjelen see, sometimes occur where lateral valleys unite with the 

 glacier-filled channels. All modern glacial lakes are of small size. One of 

 the largest lakes described in Greenland is not over three or four miles long 

 and a mile wide. Such lakes, when they exist above sea-level, are evanes- 

 cent. Mr. H. Topham described some glacial lakes of Alaska which dis- 

 charge by a tunnel, eight miles long, under 500 feet of ice. Mr. I. C. 

 Russell makes similar reports. The outflowing waters enlarge the tunnels, 

 thereby draining the lakes; but the ice roofs fall in, and by the accumu- 

 lation of ice blocks the tunnel becomes temporarily obstructed, causing the 

 water of the lakes to rise. In the very nature of the case, large lakes could 

 not be expected, for the conditions which Avould permit their formation would 

 cause the glaciers to recede. Especially would this be the case if the glaciers 

 were hundreds of feet above the sea, w^ith rivers draining beneath or through 

 them. It would be diflicult to conceive how any water-level could be main- 

 tained long enough to permit the waves to carve out terraces and sea-cliffs. 

 With glaciers coming down into the sea, it is easy to understand how bays 

 and inlets could be obstructed by the ice so as to allow the water to be 

 freshened. In such lakes the water-level could be maintained long enough 

 to leave inscriptions in the form of terraces and beaches. 



Such is a brief account of the natural history of glacial dams. It has 

 been said that the easiest explanation of the theory of our Great Lakes is 

 by regarding them as formerly great glacial dams : so it was thought ten 



