Page:Popular Science Monthly Volume 45.djvu/56

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marvelous fact of lake distribution. Prof. Bonney passes it by with the remark that there is a perfect gradation pf lakes from the smallest tarns to those of North America and Central Africa; and Mr. Douglas Freshfield says that wherever on the surface of our globe there are heights there must be hollows; and other writers think that lakes are general results of the process of mountain-making. But none of these writers have apparently even noticed the fact that glacier valley lakes have a distinctive character which separates them broadly from the lakes of all non-glaciated countries, and that they are totally absent from such countries.

But besides the mountains which possess true valley lakes, there are a number of ranges which have been glaciated yet do not possess them, and this absence of lakes has been used as an argument against the connection of valley lakes with glaciation. A little examination, however, shows us that these cases greatly strengthen our argument. Comparatively large and deep valley lakes are the result of excessive glaciation, which has occurred only when conditions of latitude, altitude, and moisture combined to produce it. In regions where glaciation was of diminished intensity, from whatever causes, valley lakes diminish in size and number, till at last only tarns are found in moderately glaciated districts. Thus, the Pyrenees were far less severely glaciated than the Alps; they consequently possess no large valley lakes, but numerous small high lakes and tarns. As we go eastward in the Alps, the diminished rain and snowfall led to less severe glaciation, and we find the valley lakes diminish in size and numbers till far east we have only tarns. The Carpathians have no valley lakes, but many tarns. The Caucasus has no lakes and very few tarns, and this may be partly due to the steepness of the valleys, a feature which is, as we shall see, unfavorable to lake formation. In the South Island of New Zealand the lakes are small in the north, but increase in size and number as we go south where the glaciation was more intense. These numerous facts, derived from a survey of the chief mountains of the world, are amply sufficient to show that there must be some causal connection between glaciation and these special types of lakes. What the connection is we shall inquire later on.

The Conditions that favor the Production of Lakes by Ice Erosion.—Those who oppose the production of lake basins by ice erosion often argue as if the size of the glacier was the only factor and urge that, because there are no lake basins in one valley where large glaciers have been at work, those which exist in another valley where the glaciers were no larger, could not have been produced by them. But this by no means follows, because the production of a lake basin depends on a combination of