76 CHICAGO ACADEMY OP SCIENCES. 



depth of several feet. Excavations have shown, however, that till 

 usually sets in at a depth of lo to 20 feet or less (i). 



In Indiana this beach, like the present shore of Lake Michi- 

 gan, is very sandy. Its dunes, however, seldom reach a greater 

 height than 50 feet, or but one-third to one-quarter the height of 

 dunes on the present shore. Wells along it have occasionally en- 

 countered' a bed of gravel at the base of the sand at levels, corre- 

 sponding with the gravelly beaches of the Illinois portion. 



One of the best exposures of this beach in the Chicago area 

 is found at the border of the campus of the Northwestern Uni- 

 versity at Evanston. The following sections, one taken by Dr. 

 Oliver Marcy, of Northwestern University, in 1864, at which time 

 there was a peculiarly good exposure, the other taken by the 

 writer in 1887, at which time there was a less extensive exposure, 

 show a slightly different section. The beach in this interval had 

 suffered an erosion of perhaps 75 or 100 feet. 



Section of Beach at Evanston made in 1864. 



I. 



Surface soil, sandy ...••... 1% feet 



2. Brown sand and fine gravel 2V2 



3. Coarser gravel, stratified. . . • • • . . 2% 



4. Fine sand • • 2 



5. Gravel, containing bones of deer. ...-.•• i 1-3 



6. Fine sand, containing oak logs. • • . . 1V2 



7. Peat or carbonaceous earth with a marl bed containing 



moUuscan shells in the lower portion or interstratified 

 with the peat • • 1V2 



8. Gravel 3% 



9. Humus soil, with stumps and logs (coniferous) • • ■ • V2 



10. Yellow clay, laminated and contorted, containing pockets of 



gravel . 3% 



11. Blue, pebbly clay 2 



Height of bluff ....... 22 feet 



Section of Beach at Evanston in 1887. 



Feet. Inches. 



1. Yellowish-red, iron-stained sand.. 3 to 5 



2. Band of bog iron ore, granular 4 to 6 



3. Gravel, with beds of sand inckided (the stratifica- 



tion is very irregular in thickness and assorting 



very imperfect) 5 to 7 



(1) The writer is indebted to Mr. W. C. Holden of the U. S. Geological Survey 

 for many data concerning the distribution of the sand and for access to the accurate 

 maps of the Pleistocene deposits in Chicago, wiaioh he has prepared for the Survey. 



