112 ILLINOIS ACADEMY OF SCIENCE 
also applied to dunes formed upon an old beach which from the 
subsidence of the lake level finds itself much above the level of 
existing waters. 
The beach dunes are characteristic of depositing shores and 
to this class are to be referred those of the southern end of Lake 
Michigan, while perched dunes are more characteristic of erod- 
ing shores and are frequent along the east shore of Lake Mich- 
igan, ranging from 10 to 400 feet above the present level of its 
waters. 
While it is evident that the greater part of the shore dunes 
of Michigan belong to one or other of the types just mentioned, 
it is conceived that dunes may also have been formed fromsandy 
material deposited by the action of ice and water about the 
margin of the Pleistocene ice sheet and afterwards worked over 
by the wind. Dunes of such an origin may be termed terrestrial 
in contrast to those of lacustrine formation that derive their 
material directly from the lake. It seems probable that some 
of the interior dune areas of Michigan are of this type and 
recently evidence has been obtained that at least a few of the 
shore dunes are terrestrial rather than lacustrine in their 
origin. 
THE NIPISSING BEACH DUNES 
The beach developed by the water of the Nipissing Great 
Lakes has been carefully traced by Leverett and Taylor.1 Upon 
portions of this beach sand dunes have been subsequently de- 
posited, although on account of the comparatively recent date 
of its development, they are nowhere very extensive. At a 
point near the north end of the bar separating Portage Lake 
from Lake Michigan, a gravel ridge between 15 and 20 feet 
above the present lake is partially uncovered just back of a low 
fore-dune and again just north of Point Betsie lighthouse is a 
similar ridge. From their height and general character it 
seems probable that these beaches are Nipissing. On either 
side of the mouth of the Platte River the Nipissing beach does 
occur, extending for several miles, covered in many places by 
low grassy dunes, but none of these appear as interesting as 
some development upon the Fox and Beaver Islands. 
1Lev verett, Frank, and Txvlor, F. B.. The P‘eistocene 0* Indiana and Men and the 
history of the great lakes. U.S. Geol. Surv. Monograph 53: 447-463. 1915 
