Eleventh International Geological Congress. 29 



predestined by the pre-Quaternary act of crushing. The ice sheet's 

 power of glaciation, working during the whole length of the ice age, 

 is said to have been unable to obliterate or essentially change the 

 surface topography; and only put its stamp upon the landscape by 

 grinding off the proximal ends of the rock edges to the well-known 

 ice-worn "round-rocks." 



The origin of giant-kettles or pot holes is explained through the 

 characteristic corrosion by sub-glacial rivers where such currents 

 pushed forward by a strong hydrostatic pressure passed over rock 

 ledges so as to form sub-glacial whirls. The assumption frequently 

 made that these kettles were formed by water falling down through 

 crevasses is found untenable, at least when applied to the many 

 low-lying kettles, which at the time of their formation, were situated 

 at a considerable depth, even as much as three hundred and twenty- 

 five to five hundred feet below the surface of the sea. In such cases 

 even the crevasses must evidently have been filled to at least the 

 same height by standing water, and it does not seem likely that a 

 water fall could bore out holes in the bed rock through such a depth 

 of standing water. The extraordinarily strong rapids of the sub- 

 glacial rivers were no doubt more than sufficient to produce even 

 giant's kettles of such imposing dimensions as those sometimes 

 found in Bohnslan, some ten to twenty feet in diameter. 



The late glacial recession of the ice proceeded at a much greater 

 speed on the continental than on the oceanic side of Sweden. Thus 

 the recession of the ice border through Bohnslan and the adjoining 

 part of the province of Dalsland took the same amount of time as the 

 whole recession from the south end of Sweden up almost to the 

 Aland archipelago, i. e., over three thousand years. 



By means of the deposits of fine glacial clay carried by sub- 

 glacial rivers out into the advancing ocean as the ice retreated, it is 

 ascertained that the late glacial sea covered a large part of Denmark 

 and extended to central Sweden and Norway, and that there were 

 important changes of land and sea level, amounting to hundreds of 

 feet. There were for example great shell deposits of shallow water 

 forms laid down where the water had been previously more than 

 three hundred and twenty-five feet deep. 



A careful study of the layers of clay and sand deposited in the 

 late glacial sea has furnished a most accurate measure of the chron- 

 ology of the retreat of the ice sheet. Baron De Geer points out 

 that, while there were many variations in the rate of recession of 

 the ice margin, the average rate of retreat in the vicinity of Stock- 

 holm was two hundred meters per year. South of Stockholm for 

 some distance the rate was from twenty to one hundred meters per 

 annum. At some points to the north, notably near Dai's Ed near 

 lakes Venern and Vettern, the ice border remained stationary for 

 one hundred or two hundred years. 



Baron De Geer's estimate of the lapse of time since the retreat 

 of the ice at Stockholm is about seven thousand years. Farther 

 north the ice lasted until a much later period; and the northern pari 

 of Sweden was probably covered with ice only two thousand years 

 ago. 



It may be remarked in passing that this estimate compares well 



