The ice age in North America and its bearing upon the antiquity of man5th edwith many new maps and illus., enland rewritten to incorporate the facts that bring it up to date, with chapters on Lake Agassiz and the Probable cause of glaciation . lso that, after the analogy of ariver, a glacier shoves sand and gravel and bowlders under-neath it along its bed ; from which it can easily be seen thata glacier is a powerful eroding agency, rasping down thesurface over which it moves, and by the firm gras]) in whichit holds the sand, gravel, and bowdders underneath it, \>yo-ducing grooves and scrat
The ice age in North America and its bearing upon the antiquity of man5th edwith many new maps and illus., enland rewritten to incorporate the facts that bring it up to date, with chapters on Lake Agassiz and the Probable cause of glaciation . lso that, after the analogy of ariver, a glacier shoves sand and gravel and bowlders under-neath it along its bed ; from which it can easily be seen thata glacier is a powerful eroding agency, rasping down thesurface over which it moves, and by the firm gras]) in whichit holds the sand, gravel, and bowdders underneath it, \>yo-ducing grooves and scratches and polished surfaces on therocks below, while these stones themselves will in turn besciatched and polished in a peculiar manner. Wherever theglaciers have receded, so that their bed can be examined,these phenomena, whi;-h we reason from the nature of thecase must have been produced, are fonnd actually to occur,and a terminal moraine is sure to cont:nn many pebbles and 12 THE ICE AGE IN NORTH AMERICA. bowlders bearing marks of the peculiar attrition to whichthey have been subjected in their motion underneath theice. The rocks brought along ui)on the surface of the gla-cier of course are not thus striated, aud ordinarily the mate-. FiG. n.—Glacial scorings (after Agassiz). rial of the kames has been so much rolled by water that ifthe pebbles ever w-ere scratched, the marks have been this brief account of the physical characteristics ofice, and of the effects produced by its movement in a gla-cier, we are prepared to enter more understandingly upon asurvey of the actual facts relating to the past and presentextent of the ice-fields over the northern part of NorthAmerica. Keserving the discussion of theories concerningtlie cause and date of the glacial period to the latter part, ofthe treatise, we will first consider the facts concerning theglaciers still existing in America, and then briefly, by way ofcomparison, those concerning glaciers in other portions oft
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