Baised Beaches of Lal-e Michigan. 185 



Lower beach which is described later, and it was observed beneath this 

 lower ridge in the same situation as beneath the ridge just described. The 

 writer has not been able thus far to satisfy his own mind with any theory 

 explaining the burying of this peaty surface beneath these lake beaches 

 If the beaches were composed entirely of sand, their material might have 

 been drifted beyond the lake border and covered the adjacent lands, but there 

 is here a heavy deposit of coarse gravel capping the beach, which certainly 

 could not be drifted by the wind. It might seem easy to account for the 

 phenomena by supposing that thei'e have been oscillations of level with 

 periods of emei'gence followed by periods of sabmergence, but we found 

 no evidences of such oscillations elsewhere. The limitation of the peat to 

 the immediate border of the beach also presents difficulties in forming au 

 explanation under hypotheses which have thus far arisen. The writer has 

 attempted to conceive a method by which a protective bar could have been 

 built up into the form which this beach line presents, but has not been suc- 

 cessful. 



Continuing now the distribution and description of the Middle beach 

 from the Chicago river southward, we find that the western Middle beach 

 repeats the phenomena of the Upper beach in swinging down the north 

 side of the Chicago river so that when we cross the river to find its con- 

 tinuation we must pass up stream some two miles. The Middle beach is 

 well developed immediately north of Jefferson, in Sec. 9, Tp. 40, R. XIII E. 

 It has here an elevation of 40 feet above Lake Michigan, and is therefore 

 as elevated as the portion of the beach immediately opposite, and some 

 5 — 10 feet higher than the spur referred to above as extending eastward 

 along the north side of the river. 



This beach hue has a course varying slightly to the east and west of 

 south through Cragin's and Austin to the south part of Sees. 16 and 17, 

 Tp. 89, R. XII [ E., when it assumes a southeasterly course to the Des 

 Plaines river, at Riverside. The following are the elevations of the crest 

 of this beach at points crossed by railway lines: Jefferson, 40 feet; 

 Cragin's, 38 feet; Austin, 38 feet; Wisconsin Central crossing (west side of 

 Sec. 16, Tp. 39, R. XIII E.) 38 feet; Riverside, 40 feet. 



This portion of the beach line between the Chicago and Des Plaines 

 rivers seldom rises more than 6 — 7 feet above the plain, which borders it 

 on the east, and but 2 — 3 feet above that on the west. It is a low beach, 

 coated by beach gravels 1 — 2 feet in depth, with an occasional depth of 

 6 — 8 feet. This bencli is a noticeable feature on the level plain, though its 

 elevation is so shght, and its course is well shown by a narrow belt of tim- 

 ber, which follows it across the otherwise treeless prairie. 



Between Riverside and Summit is the old outlet of Lake Michigan into 

 the Des Plaines valley. At Summit there is a prominent beach line turn- 

 ing down the east bluff of the Des Plaines river, which traced eastward 

 from this opening into the river valley, takes a more or less direct course 

 toward the nortli end '• Blue island till ridge," and keeping slightly uortli 



