M. L. FULLER 



Fig. 



Figs. 



Fig. 4. 

 3 AND 4. — Showing mode of formation of 



gradational deposits: 3, marginal deposits before reces- 

 sion of ice; 4, plain and gradation deposits after recession 

 of ice. 



were laid down by the superglacial streams from the surrounding ice. 

 The sand and gravel plains are most perfectly developed along the 

 hillsides, as the lakelet was there shallowest and soonest free from 

 ice. In such situations the plains were frequently built up to a hori- 

 zontal upper surface, coinciding approximately with the water level, 



and are generally free 

 from kettles. As the 

 ice receded, lower por- 

 tions of the hillsides 

 were uncovered, and 

 the water became 

 deeper. The ice, how- 

 ever, melted back most 

 rapidly along the sur- 

 face of the lakelet, 

 leaving projecting 

 edges beneath the 

 water, which became 

 covered with sands and gravels. On the further melting of the ice 

 and its disappearance from beneath these gravels, the materials were 

 let down into irregular accumulations along the sloping valley sides, 

 constituting the gradational deposits between the upper and lower 

 plains, and between the lower plains and the present valley floor in the 

 Stoughton Bay area. When the ice-wedge was very thin, gentle and 

 fairly regular slopes resulted when the materials were let down, but 

 when thicker, steeper and kettle-pitted slopes resulted. Dry Pond, 

 in the southern part of East Sharon, is an example of such a kettle. 

 This is the explanation of the change from a fiat to a gently sloping 

 plain, and finally to the irregular hummocky slopes which characterize 

 the plains at many points in this area. The steeper ice-contact slopes, 

 which are especially well developed in portions of the east side of the 

 East Sharon plain, were formed where the marginal ice-wedge was 

 only slightly developed. That the slopes of the plains cannot be 

 regarded as purely depositional is shown by the fact that the inclina- 

 tion of the surface is opposite that exhibited by delta plains. 



There can be no question that the materials of the plains came 

 from the ice. This is attested by the rounding of the pebbles, the 



