360 Influence of Water and Ice on the Earth's Features. 



tain to tlie sea. But these objections, though fair and power- 

 ful, are not unanswerable. Though it is true that some glaciers 

 do ride over moraines, it is equally true that other glaciers cut 

 out hollows and remove natural obstacles. The snout of a 

 glacier, too, is its weakest part, not its strongest. The same 

 glacier which, in some parts of its course, presses with a weight 

 of ten tons on the square foot, and moves at the rate of twelve 

 inches per day or more, will at another part near its extremity 

 have scarcely a calculable motion and an extremely small pres- 

 sure. The excavation will be made where the moving force is 

 greatest, and the deposit where it is smallest. Thus a glacier 

 may easily be pushed up-hill over its own moraine. The lake 

 basin need not be excavated by the extremity, but rather by 

 the body of the ice; and lakes not in rocky basins may be and 

 are occasionally formed in this way. 



The excavating effect of ice is not to be looked for on the 

 hardest but on the softest part of the surface it travels over. 

 It is when a gigantic mass of ice, coming from a great height, 

 and pressed onwards by enormous weight, traverses a wide 

 and comparatively flat space, crossing it entirely, and dying 

 away at a distance beyond, that we must look for the chief 

 effect; not when the ice merely enters the valley, and is melted 

 at the first contact with it. 



And, thirdly, the emergence of the ice from a deep hollow, 

 even of 2000 feet or more, ceases to be impossible, or even 

 difficult, when the distance is sufficient. The deepest part of 

 the Lago Maggiore is about 2600 feet, and the distance to the 

 outflow of the lake twelve miles. The rise is, therefore, one in 

 twenty-four, or an angle of 2° 21', which is a slope that could 

 hardly be recognized by the most practised eye. The case of 

 the Lake of Geneva, which has been mentioned, is another 

 example ; but the depth there is not half as great, and the dis- 

 tance more than double. This is an angle of 25', and is prac- 

 tically horizontal. 



At any rate, it is certain that the great lakes of the world 

 have not been proved to lie in the axis of geological dis- 

 turbances, either in faults or synclinal lines. Many of the 

 most important, as Lake Superior and Lake Ontario, certainly 

 do not. And how, then, can they be explained ? The mere 

 action of moving water can hardly plough out the bottom even 

 of a shallow lake. In limestone districts, water running un- 

 derground may sometimes eat away the rock, and the roofs of 

 a succession of caverns falling in may produce a kind of chan- 

 nel; but these cases are limited, and certainly would not account 

 for many lakes of large size. Even if they follow lines of least 

 resistance, the whole excavation has been by ice or water, and 

 of the two ice affords the most reasonable explanation. 



