128 ORIGIN OF TILL. [Ch. XI. 



frozen into it. In order to understand in what manner long and straight 

 grooves may be cut by such agency, we must remember that these float- 

 ing islands of ice have a singular steadiness of motion, in consequence 

 of the larger portion of their bulk being sunk deep under water, so that 

 they are not perceptibly moved by the winds and waves even in the 

 strongest gales. Many had supposed that the magnitude commonly 

 attributed to icebergs by unscientific navigators was exaggerated, but 

 now it appears that the popular estimate of their dimensions has rather 

 fallen within than beyond the truth. Many of them, carefully measured 

 by the officers of the French exploring expedition of the Astrolabe, were 

 between 100 and 225 feet high above water, and from 2 to 5 miles in 

 length. Captain d'Urville ascertained one of them which he saw float- 

 ing in the Southern Ocean to be 13 miles long and 100 feet high, with 

 walls perfectly vertical. The submerged portions of such islands must, 

 according to the weight of ice relatively to sea-water, be from six to eight 

 times more considerable than the part which is visible, so that the mechan- 

 ical power they might exert when fairly set in motion must be prodigious.* 

 A large proportion of these floating masses of ice is supposed not to be de- 

 rived from terrestrial glaciers, f but to be formed at the foot of cliffs by 

 the drifting of snow from the land over the frozen surface of the sea. 



We know that in Switzerland, when glaciers laden with mud and stones 

 melt away at their lower extremity before reaching the sea, they leave 

 wherever they terminate a confused heap of unstratified rubbish, called 

 " a moraine," composed of mud, sand, and pieces of all the rocks with 

 which they were loaded. We may expect, therefore, to find a formation 

 of the same kind, resulting from the liquefaction of icebergs, in tranquil 

 water. But, should the action of a current intervene at certain points or 

 at certain seasons, then the materials will be sorted as they fall, and ar- 

 ranged in layers according to their relative weight and size. Hence there 

 will be passages from till, as it is called in Scotland, to stratified clay, 

 gravel, and sand, and intercalations of one in the other. 



I have yet to mention another appearance connected with the boulder 

 formation, which has justly attracted much attention in Norway and other 

 parts of Europe. Abrupt pinnacles and outstanding ridges of rock are 

 often observed to be polished and furrowed on the north side, or on the 

 side faciug the region from which the erratics have come ; while, on the 

 other, which is usually steeper and often perpendicular, called the " lee- 

 side," such superficial markings are wanting. There is usually a collec- 

 tion on this lee-side of boulders and gravel, or of large angular fragments. 

 In explanation we may suppose that the north side was exposed, when 

 still submerged, to the action of icebergs, and afterwards, when the land 

 was upheaved, of coast-ice which ran aground upon shoals, or was packed 

 on the beach ; so that there would be great wear and tear on the sea- 

 ward slope, while, on the other, gravel and boulders might be heaped up 

 in a sheltered position. 



Northern origin of erratics. — That the erratics of northern Europe 

 * T. L. Hayes, Boston Journ. Nat. Hist. 1844. f Principles, ch. xv. 



