130 MOUNTAIN AND MOORLAND 



which are laid down in nature so as to form dams are 

 very varied; they may be due to moraine materials 

 and boulder clay which glaciers have brought down, 

 or to debris from the mountainside swept down in a 

 flood, or to landslips and screes, or to a stream of 

 lava blocking a valley, or to other causes. A lake 

 sometimes arises in a crater that is not too leaky, and 

 a very interesting kind of lake has sometimes been 

 formed by beavers building a dam across a river. It 

 might be thought that many of these dams would be 

 too full of holes to keep back the water, but we may 

 notice on the seashore that a bank of sand thrown up 

 in a storm often forms a sufficient dam for a lake 

 between it and the cliffs or the dunes. This is due to 

 capillary attraction holding the water between the 

 grains of sand, and we have sometimes seen a 

 temporary dam greatly helped by bands of seaweed. 

 Moreover, while the dam made by moraine materials 

 or by earth carried down from the hill by a flooded 

 stream may be at first rather leaky, the percolating 

 water soon shuts up its own way of escape by 

 depositing clay and fine particles in all the little 

 interstices. Windermere and Ullswater may be men- 

 tioned as probable examples of lakes mainly due to 

 barriers of boulder clay (the material accumulated 

 underneath a glacier) blocking a valley. 



The second way in which lakes may be formed is by 

 the erosion and solution of hard rock. A glacier 

 filling a river valley may gouge out a deep hollow, 

 and sometimes it is plain that the ice and the rock 

 materials brought with it must have risen over a very 

 hard rock barrier and gouged out a second deep 

 hollow lower down the valley. This has been the case 



