10 BULLETIN 141, U. S. DEPARTMENT OF AGRICULTURE. 



the deeper and quieter lake waters in positions more remote from the 

 deltas of tributary streams and at some distance from shore-line 

 borders. They usually rest either upon consolidated rock, upon 

 glacial till, or upon the more sandy or gravelly materials which 

 were sometimes deposited first as the ice retreated. 



Thus- the greater part of all of the glacial lake basins consists of 

 marginal gravelly and sandy zones, of local sand plains and stream 

 deltas, and of the heavier loam and clay deposits more remote from 

 the sources of sedimentary supply. 



When the waters of the different glacial lakes were gradually 

 withdrawn and the bottoms of these lakes exposed to form a land 

 surface there were many minor inequalities of elevation which gave 

 rise to wide differences in the drainage features of the lake basins. 



Areas lying between successive beaches frequently remained 

 swampy. Shallow depressions in the broader lake plains still con- 

 tained minor ponds and swamps. Only the higher lying areas and 

 the more sloping surfaces became well drained immediately after the 

 recession of the glacial lake. The broad level areas, occupied by the 

 heavier clays and loams, together with all depressed areas within the 

 glacial lake plains, remained swampy for a considerable period of 

 time. In consequence large areas included Avithin the glacial lake 

 basins passed through a swampy stage which persisted in many 

 instances until the occupation of the land by white settlers, and 

 which has only been relieved to a partial extent through the installa- 

 tion of artificial drainage. 



It is probable that water-loving grasses and the smaller forms of 

 vegetation first occupied these swampy areas. It is certain that con- 

 siderable areas of the swamp included within the glacial lake basins 

 remained so poorly drained until within historic times that only a 

 few species of trees found foothold within their limits, while in many 

 instances considerable areas remained in the condition of treeless 

 marshes or grass-grown swales. 



In other instances areas somewhat better drained eventually became 

 covered with a heavy stand of ash, elm, soft maple, tamarack, and 

 other water-loving trees. In all cases the swampy conditions gave 

 rise to the formation of large amounts of humus in the surface soil 

 and this has given a characteristic dark gray, brown, or black color to 

 the surface layer of extensive areas of the glacial lake deposits in the 

 north-central and northeastern States. These give rise to the soils of 

 the Clyde series. Conditions of more perfect drainage gave rise to 

 light-gray or yellow surface soils which are classed in other soil series 

 than the Clyde. 



The different soils of the Clyde series, therefore, owe their origin 

 to a complex series of events beginning with the glaciation of the 

 northeastern and north-central States, followed by the retreat of the 



