AGE OF HUMAN RACE EICHARZ 459 



THE DURATION OF THE ADVANCE OF THE ICE 



To determine tlie time neodeil for such a readvance no exact 

 methods are at hand. We meet here with many uncertainties and 

 unknown quantities which prevent an exact measurement. More- 

 over, we do not know how lon<^ the ice front stopped at the greatest 

 extension of the inhmd ice, and how long man was in Europe before 

 the ice started its readvance. Only an estimate of tiie readvance of 

 the ice will be attempted in the following paragraphs. 



The average motion of recent glaciers and ice fields is fairly well 

 known. The Alpine glaciers move at a daily rate of 12 to 20 inches. 

 There are examples of more rapid motion; this happens where a 

 glacier is pushed with high pressure into a narrow valley or where 

 the ice slides over a steep slope. Such conditions are, for example, 

 responsible for the exceptional velocities of glaciers at the western 

 edge of the large ice sheet of Greenland, where daily motions of 

 'AS to 66 or even of 105 feet are recorded. It would be wrong to 

 attribute such a speed to the whole body of the inland ice. The 

 late Prof. Thomas C. Chamberlin long ago warned against such 

 a procedure : " There is a widespread misapprehension as to the 

 average rate of movement of the ice fields of Greenland. * ♦ * 

 In certain fiords, that lead out from great basins into which broad 

 fields discharge their ice and their surface waters, and thus furnish 

 the conditions for an extraordinary rate of movement, the rate of 

 motion, at least during the summer, is unusually high, and these 

 exceptional cases have been taken as representative of the movement 

 of the border of the inland ice. This is ver}' far from being true. 

 The average movement for the whole border of the ice field is quite 

 certainly less than 1 foot per day, and it is more likely less than 1 

 foot per week." ^^ Others speak of a forward motion of only a few 

 meters annually. If it were more, the ice would soon cover those 

 j)arts of Greenland which for centuries have been free from ice, the 

 wastage of the ice being exceedingly small on account of the short 

 melting season. 



The Pleistocene ice sheets were of enormous thickness, and they 

 advanced, as a rule, over a flat country, spreading out over a very 

 large area. The surface slope, which is of the greatest importance 

 for the rate of movement, has been found to be very slight, as far 

 as can be ascertained. All this points to a slow motion of the 

 clumsy ice masses, although an exact rate can not be given. The 

 southernmost portions of the ice may have moved faster than the 

 Greenland ice because, reaching into an area of milder climate, the 

 greater part of the ice was not far from its melting point. Conse- 



"T. C. ChamborlalD and R. D. Salisbury, Geology, 2d ed., 1907, Vol. Ill, p. 430. 



