EPEIROGENIC MOVEMENTS EFFECT ON ICE AGE \f 



Mississippi basin, and on the present areas of the North and Baltic seas, north- 

 ern Germany, and Russia. 



Late Glacial or Champlain Depression known by fossiliperous marine 

 Deposits overlying the Glacial Drift 



NORTH AMERICA 



Under their ice-burden, these continental areas finally were de pressed, in the 

 later part of the Glacial period, until they mostly stood somewhat lower than 

 now. The coastal submergence of New Hampshire and Maine ranged from 100 

 to 300 feet, as is known by fossiliferous marine beds there overlying the drift. 

 In the Saint Lawrence valley the depression of the land below its present height, 

 proved by the same kind of evidence, attained a maximum of at least 560 feet 

 at Montreal, and it was about 400 feet in the northern part of the basin of 

 lake Champlain"; but it decreased southward, so that the Hudson valley and the 

 basins of lake Ontario and the upper Laurentian lakes were above the sealevel. 

 Around Hudson bay and south westward to the area of the glacial lake Agassiz the 

 epeirogenic downward movement of the ice-laden continent reached its limit mostly 

 about 500 to 600 feet below the present elevation. All the area of the North Amer- 

 ican ice-sheet was similarly lowered from the great altitude to which it had been 

 uplifted, so that in the late part called the Champlain epoch of the Glacial period 

 the borders of the icefields were subjected to climatic conditions of warm and even 

 hot summers nearly like those prevailing now in the same latitudes. 



The Champlain depression of the British isles appears to have had a maxi- 

 mum of about 100 to 300 feet below the present sealevel in Lancashire and Scot- 

 land ; but marine shells glacially transported from early Pleistocene sea beds 

 occur at much greater altitudes up to a maximum of 1,300 feet on Moel Tryfan, in 

 northern Wales. The limit of the depression of Sweden and Norway was mostly 

 between 100 and 600 feet ; but its highest raised shoreline on the west side of the 

 gulf of Bothnia exceeds 800 feet, and Barou De Geer computes the maximum sub- 

 sidence of the interior of the peninsula as about 1,000 feet. All the glaciated area 

 of Europe, like that of our own continent, sank in the Cliamplain epoch from the 

 preglacial uplift to its present height, or lower, and has since been nearly un- 

 changed in altitude or has experienced oscillations of moderate re-elevation. 



Competence op the Champlain Depricssion to terminate the Ice Age 



Although the adequacy of the preglacial epeirogenic uplift of this continent to 

 produce its Pleistocene ice-sheet was tardily recognized and can not yet be said to 

 be accepted by all American glacialists, it was distinctly claimed by Dana, in 1870, 

 that the Champlain subsidence of the land beneath its ice-load, supposing it to 

 have been previously at a high altitude, must have brought climatic conditions 

 under which the ice would very rapidly disappear. This subsidence, however, 

 probably afi"ected the whole of the preglacially elevated areas, on each side of the 

 North Alantic, before the growth of the ice-sheets was checked. For somie time 

 the increase in the ice accumulation may have exceeded the rate of depression, so 

 that the surface of the thickening ice-sheets continued to hold an undiminished 

 altitude. But at length the subsidence brought a warmer climate on the borders 

 II— Bull. Geol. Soc. Am., Vol. 10, 1898 



