V CxLACIAL EROSION OF LAKE BASINS 107 



superficial beds, often causing the wildest confusion in the 

 deposits and sometimes embedding huge sheets of tertiaiy 

 strata or chalk in the midst of the boulder-clay. But this 

 is a very different mode of action from that by which hard 

 rocks are ground down or lake-basins eroded. 



In reply to the continual assertions of Professor Bonne}' 

 and of most of the Alj^ine explorers, that the action of 

 glaciers is entirely sujDerficial, and that they actually pre- 

 serve the surfaces they cover from denudation, a few facts 

 may be here given. From a large number of gaugings by 

 Dollfus-Ausset, Dr. Penck has calculated that the solid 

 matter in the torrent which issues from the Aar glacier 

 annually amounts to six hundred and thirty-eight cubic 

 metres for each square kilometre of the surface of the 

 glacier, a quantity sufficient to lower the bed of the glacier 

 one metre in sixteen hundred and sixty-six years, or one 

 foot in five hundred and 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 favourable to ice 

 as an agent of erosion ; but even if the amount annuall}' 

 ground away be but small, the cumulative effect was un- 

 doubtedly very great in the case of the enormous glaciers 

 of the ice age. The very wide areas covered with boulder 

 clay and drift in North America, and its great average 

 depth, have already been referred to in the preceding 

 chapter (p. 75) ; but a still more striking estimate has been 

 made of the amount of rock debris 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 

 debris 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 coun- 

 tries. To this must be added the amount which has gone 



' Falsan, La Periode Glanaire, p, 90. 



