GEOLOGICAL AND NATURAL HISTORY SURVEY. 73 



marshy. Notwithstanding the presence of this bar the beach back 

 of it seems to have been acted upon by lake waves with nearly 

 as much vigor as the portion of the beach farther south, which 

 was not thus protected. The bar is much stronger than any part 

 of the beach proper, being lo to 20 feet in height and nearly one- 

 fourth mile in average breadth — if the sand and gravel on its 

 borders are included. 



The second beach is, on the whole, characterized by larger 

 deposition than the upper one, there being throughout much of its 

 extent a line of sandy gravel, 5 to 10 rods or more in width and 

 2 to 5 feet in height, but it has less conspicuous cut terraces. Its 

 strength is greater in the vicinity of the northern outlet and on the 

 north side of the southern outlet than at points between or outside 

 the outlets. In Illinois and Michigan, however, it is heavily cov- 

 ered wath sand, which is probably in large part wind drifted. These 

 accumulations of sand in places reach the height of 30 to 40 feet. 



Occasional reports of the discovery of a molluscan shell in 

 this beach have come to m}^ notice, but none have been personally 

 noted. In this respect it is similar to the Upper beach, but 'in 

 striking contrast with the next lower beach, which is full of fossils. 



An excellent exposure of the structure of the bar noted above 

 is found immediately north of Evanston, where the lake is under- 

 mining the bar as well as subjacent deposits. The beach sands 

 and. gravels rest upon a bed of peat, which was noted by Dr. 

 Andrews and interpreted by him to be the accumulation of a marsh 

 or partially summerged land surface. The peat not only underlies 

 the bar under discussion, but extends eastward across the interval 

 between it and the Third beach. Its level is no higher than that of 

 the Third beach, being but 12 to 15 feet above the present level of 

 Lake Michigan. The peat is in places several feet thick, but at the 

 point where the bar comes out to the lake shore it has a thickness 

 of only 3 to 6 inches. It contains pieces of mangled wood and has 

 been disturbed by waves. Between the peat and the yellowish blue 

 till, which forms the base of the exposure, there is a gravelly sand 

 6 to 18 inches in thickness, which appears to be a lacustrine deposit. 

 The peat is immediately overlain by about 5 feet of sand, above 

 which there is a bed of coarse gravel. The gravel is thin near the 

 borders of the bar, but has a thickness of 11 to 12 feet beneath the 

 highest part. It is capped by a thin deposit of sand and has layers 

 of sand interstratified with it in its thickest part. The presence of 

 this gravel makes it impossible to suppose that the old land surface 

 has been buried by the drifting of material from the lower beach. 



