EROSION AND DEPOSITION IN REGION OF GREAT LAKES 727 



STUDY OF ICE-SHEET EROSION AND DEPOSITION IN THE REGION OF THE 



GREAT LAKES 



BY FRANK BURSLEY TAYLOR 



(Abstract) 

 The paper describes: 



(1) The distribution of the drift with reference to its thickness in the 

 southern peninsula of Michigan in some detail and in adjacent and more dis- 

 tant parts of the Great Lakes region in less detail. 



(2) The disposition of the larger thick and thin areas with reference to the 

 major elements of topography, and also with reference to the direction of ice- 

 movement and the principal currents and eddies of the ice-sheet. 



(3) Erosion by the ice-sheet in several limited localities is described, some 

 representing maximum, others medium, and still others minimum effects, and 

 the relation of these effects to the major features of topography and to the 

 principal ice currents and eddies are discussed. The conclusions thus reached 

 are generalized and extended to larger areas and to the region as a whole. 



(4) The effects of the marginal oscillations of the last ice-sheet upon the 

 deposition of its drift, as modified by the arrangement of the principal cur- 

 rents and eddies of the ice-sheet and the larger element of land relief, are 

 discussed. 



(5) The conclusion is reached that erosion by the ice-sheet in the region of 

 the Great Lakes was extremely small, except on materials already weathered 

 and loosened. In the great belt of thick drift, lying principally south of the 

 Great Lakes, deposition predominated for the greater part of the time, and 

 erosion was almost nil. Maximum erosion effects occur on overridden reefs in 

 the path of the principal currents. Maximum deposition effects occur in eddy 

 areas behind large obstructing masses, along eddy lines between the greater 

 currents, and in the great thick drift belt where marginal lobes were spreading 

 and unloading and where the number of oscillatory readvances was greatest. 



(6) The further conclusion is reached that the Great Lakes basins were not 

 scooped out by the ice-sheet, and that effects of deepening and widening by 

 ice-sheet erosion are quite local and are offset or overbalanced by shallowing 

 and narrowing by deposition in other parts, which are described. 



(7) The reason for the lightness of ice-sheet erosion in the Great Lakes 

 region as compared with that in Alpine and fiord valleys is (a) lack of con- 

 centration and long maintenance of flow in narrow chutelike courses, and (&) 

 lack of the steep declivity requisite for a relatively high velocity of flow. There 

 was nothing in the Great Lakes region resembling the conditions which attend 

 the making of fiords or overdeepened Alpine valleys, where erosion by ice 

 finds maximum efficiency and power. 



(8) The valleys of the Finger Lakes are not analogous to fiords, but are 

 indentations in the infaciug rim of a basin from which the ice moved toward 

 and over the rim, and have not been made or notably deepened by the ice- 

 sheets. 



(9) The main conclusions reached agree \<^ith those of Fairchild, namely, 

 that in the region of the Great Lakes ice erosion of unweathered rock was 

 extremely slight, but they do not agree with the broader generalization Implied 



