Page:Popular Science Monthly Volume 45.djvu/62

50 five years; and the same writers calculate that the same amount of erosion in a valley by water alone would require two and a half times as long. Other writers have made estimates less favorable to ice as an agent of erosion; but even if the amount annually be but small, the cumulative effect was undoubtedly very great in the case of the enormous glaciers of the Ice age. The very wide areas covered with bowlder clay and drift in North America, and its great average depth, have already been referred to in my previous article (Popular Science Monthly, April, 1894, p. 782); but a still more striking estimate has been made of the amount of rock débris in northern Europe which can be traced to Scandinavia. Dr. Amund Helland states that about eight hundred thousand square miles are covered with such drift to an average depth of one hundred and fifty feet, of which about one hundred feet are of Scandinavian origin, the remainder being local. The area of Scandinavia and Finland, from which this débris has been derived, is very much less than the area over which it is distributed, so that to produce it an amount equal to an average thickness of two hundred and fifty-five feet must have been removed from those countries. To this must be added the amount which has gone into the Baltic and North Seas, and also that which has been carried away by rain and rivers since the Ice age passed away, and yet further, the enormous amount that still remains on the lowlands of Scandinavia, and we shall then arrive at an amount probably twice as great as the above estimate, that is, something like five hundred feet as the average amount of ice erosion of Scandinavia during the Glacial period. Now, unless this estimate is wildly and extravagantly erroneous—and Prof. Geikie adopts it as prima facie not extravagant—we have an amount of ice erosion so enormous as to put completely out of court all the allegations of those who attempt to minimize it as a mere smoothing off of sharp angles and rugged surfaces. I am not aware that Prof. Bonney denies the Scandinavian origin of the greater part of the northern drift, and unless he can show that its quantity is something like a fiftieth part only of the estimate of Dr. Helland, I can not understand how he can still maintain that the glaciers and ice-sheets of the Ice age were agents of abrasion, not of erosion, and that they were therefore impotent to grind away the comparatively small amount of rock removed, under the most favorable conditions, from the basins of the valley lakes whose origin we are discussing.—Fortnightly Review.