228 



PHYSICAL GEOLOGY 



FIG. 247. Section of 

 lake at the margin of an 

 ice sheet. 



Outwash plains. Where overloaded streams that issued 

 from the ancient ice sheet did not find valleys for their ac- 

 commodation, as was often the case, 

 they spread their material in fanlike 

 deposits in front of the ice. Many 

 a such deposits made by neighboring 

 streams often joined to form alluvial 

 plains, known as outwash plains, 

 which slope gently away from the terminal moraines which 

 they front (Plate XII). 



Deltas. Marginal lakes were sometimes formed at the 

 edge of the ancient ice sheet where the land sloped down- 

 wards toward the ice, forming a temporary basin (Fig. 247). 

 Where streams issued from the ice at the edges of lakes, 

 they deposited their loads in the form of deltas. Such deltas 

 are common in parts of New England. 



Kames. The edge of the ancient ice sheet was doubt- 

 less jagged and irregular (Why?). Subglacial streams, 

 flowing in tunnels beneath the ice, were often under great 

 pressure, like the water in a long tube. When such streams 

 issued from beneath the ice in reentrant angles of its edge, 

 the pressure, and therefore their velocity and carrying power, 

 were reduced. This caused them to make deposits, which 

 were shaped by the par- 

 tially inclosing ice walls. 

 In this way irregular 

 mounds and hillocks of 

 rudely stratified and 

 water-worn material were 

 formed in association with 

 the unstratified deposits of 

 the terminal moraine. 

 Such deposits are called 

 kames (Figs. 248 and 227). 



Eskers. Subglacial streams sometimes deposited sand 

 and gravel along the floors of the ice tunnels through which 



FIG. 248. Kame east of Kewaskum, 

 Dodge County, Wis. (Alden, U.S. Geol. 

 Surv.) 



