RESULTS ACCOMPLISHED BY THE ICE-SHEETS. 345 



accumulations, but the volume of the drift diminishes and ceases with 

 an attenuated and often indistinctly defined margin. This condition 

 seems referable to a gradually decreasing power of the ice to carry forward 

 the drift to its margin, on account of the thinness of the ice there and 

 the low gradient of its surface slo})e. Accumulation by snowfall is, there- 

 fore, more ])robable than an incursion of thick ice flowing onward over 

 new ground as the method of growth of these outer parts of the ice- 

 sheets. 



EEOSION, TBANSrOHTATION AND DEPOSITION OF DRIFT BY THE ICE-SHEETS. 



The bed-rocks on the marginal areas of thinly attenuated drift have 

 suffered little erosion. In the states of Ohio, Indiana, Illinois, Iowa, 

 northern Missouri, and eastern Kansas and Nebraska, the great area of 

 the outer and older drift on that central part of the basin drained by the 

 Mississippi river has yielded a much less amount of drift than the country 

 fartlier north. Upon large tracts no glacial stride are preserved by the 

 bed-rocks, which seem on the average to have lost scarcely more by ice 

 erosion than their mantle of preglacial valley deposits and the residual 

 clays left by the })rocesses of preglacial rock-decay on the higher lands. 

 Drift transportation and deposition, ratlier than erosion to obtain addi- 

 tional drift, were the chief work of the ice-sheet in that region. When 

 the ice accumulation by snowfall attained a thickness of several hundred 

 feet 10 to 30 miles back from its boundary, increasing northward to thou- 

 sands of feet on the region of the Laurentian lakes, northern Minnesota 

 and Manitoba, its currents carried away much drift from the last named 

 areas of plentiful rock erosion and deposited it on the broad plain 

 country covered by the comparatively thin ice accumulation in the 

 Mississippi basin. 



FORMATION OF MORAINES. 



After a consideral)le retreat of the ice-border had taken })lace under the 

 warm climate which brought the CJlacial period to its end, very different 

 conditions of ice action prevailed. Instead of the mostly smooth and 

 even surface of the earlier drift, the later and more northern drift was 

 deposited with nmch unevenness in its general contour, enclosing multi- 

 tudes of lakes in its hoHows, and including niunerous api)roximately 

 parallel or irregularly interlocking series of marginal drift accumulations 

 called moraines. These belts of hilly drift seem to mark stages when the 

 glacial recession was temi)orarily interru])ted by halts or slight readvances 

 of the generally waning ice-border. The high altitude of the ice-sheet 

 forbade extensive melting upon tlie greater part of its area, but the warm 

 climate of tlie Champlain ei)och rapidly melted its borders, j)roducing 



