lOWAN DRIFT 729 



and decomposed to considerable depths, so tbat 1 Lave seen a shaft sunk to a 

 depth of GO feet in the decomposed rock with pick and shovel alone after the 

 Irost has been driven out by heat. We might, therefore, confidently expect 

 that the schists and granites of other portions of nortliern Canada would have 

 been decomposed to this depth at least. But the Klondike region is scarcely 

 typical of northern Canada ; its hills are higher than those in most of the rest 

 of the country, and its surface is even now undergoing active atmospheric 

 erosion, so that the products of surface decomposition are being continually 

 carried down into the bottoms of the valleys. On the other hand, the surface 

 over the Archean Shield of northern Canada had, in pre-Pleistocene times, 

 been reduced almost to a peneplain, and therefore as its rocks became decom- 

 posed they would not be carried away so rapidly as on the more hilly country, 

 and the thickness of soft weathered material should have been greater than 

 on the hills of the Klondike. The decomposed rock covering the Archean 

 peneplain was removed by the ice-sheet of the first Glacial period and spread 

 out over the hills and plains around the edge of the glaciated area. Thus 

 much of the detritus removed by the first ice-sheet was soft, rotten rock, and 

 consequently in a powdered or friable condition, and not in the condition of 

 boulders or fragments of undecomposed rock. The boulders and undecom- 

 posed rock fragments, which are so plentifully scattered in and through the till 

 in the peripheral portions of the glaciated areas of North America, were 

 probably plucked off and transported from the Archean Shield and its vicinity 

 by later ice-sheets, and when some estimate can be made of the extent of this 

 undecomposed material, a good basis will have been secured for a reasonable 

 estimate of the quantity of erosive work performed by the ice-sheets of the 

 second and later Glacial periods. Mr. Taylor is quite correct in stating that 

 there is ver^^ little till in the more northern parts of Canada near the centers 

 of glaciation ; but in western Canada, at all events, there is a very wide belt 

 of country heavily covered with till within the periphery of the glaciated area. 



WW AN DRIFT 

 BY SAMUEL CALVIN 



(A1)stract) 



Three papers having more or less to say about the lowan drift have ap- 

 peared recently. Two of these express doubt as to whether there is an lowan 

 drift. The third raises the question whether, even if such a drift exists, the 

 name it has been wearing should not be applied to something else. Taking up 

 the question raised by the last of the three papers, an effort is made to show 

 that the practice of applying the names lowan and Kansan to two super- 

 Aftonian drifts, a practice followed by geologists in recent years, is the only 

 one at all consistent with the original texts and maps where these deal with 

 the composition, color, and petrological contents of the two drifts named, or 

 delineate their areal distribution. To shift the term Kansan to the sub- 

 Aftonian till and the name lowan to the first of the super-Aftonian drifts 

 would involve the re-writing of the descriptive parts of the original texts and 

 the re-drawing of the map opposite page 727 in the third edition of Geikie's 

 "Great Ice Age." It accords better with what was published at the time the 

 names were applied to let recent usage remain unchallenged and unchanged. 

 XLVIII— Bull. Gkol. Soc. Am., Vol. 22, 1910 



