MICHIGAN ACADEMY OF SCIENCE. 199 



depressions in the northern ridge, one leading to the west end of Long 

 Lake, and the other to the east end of Platte Lake; and one in the 

 southern ridge, now occupied by the outlet of Crystal Lake to the Betsie 

 River. The edges of these ridges originally extended much farther into 

 Crystal Lake,, as indicated by bluffs caused by erosion of the lake. Tliey 

 were also mucli nearer together at their western ends on Lake Michigan. 

 These ridges are composed mainly of sand and gravel, more or less 

 water-washed and stratified, and they contain at least one layer of 

 laminated clay several feet in thickness, as shown clearly by the exposure 

 on the Lake Michigan bluffs. This layer is marked on these bluffs by a 

 zone of vegetation, which is probably due to the seepage of water held 

 and brought to the surface by this clay layer. The character of the 

 surface soils varies as different layers are exposed by erosion. This 

 glacial dei^osit was apparently laid down on preglacial ridges, as indi- 

 cated by the depth of Crystal Lake, which reaches 200 feet in places, 

 and by soundings off shore in Lake Michigan which show the existence 

 of drowned valleys opposite the end of Crystal Lake, and also of Frank- 

 fort Harbor. The level areas between the ridges are due to water action 

 at the time of the Algonquin depression. 



The topography as well as the character of these deposits and the 

 identification of Algonquin beaclies, show that in Algonquin times 

 Crystal Lake was connected with Lake Michigan toward the north by 

 channels passing west of Long Lake, toward the south through 

 the valley of the present outlet, and to the west through the then 

 unfilled valley which now contains Crystal Lake. During this time a 

 broad harbor bar was built across this valley between the western ends 

 of the glacial ridges. This bar, which is approximately two miles long 

 and three-quarters of a mile wide, now cuts off Crystal Lake from Lake 

 Michigan, and accounts for the name given to this region. At present 

 the western outline of the bar is convex, but probably when first formed 

 the glacial ridges extended much farther into Lake Michigan, and as 

 they have been cut back the bar assumed its present shape. The region 

 is still being eroded, as shown by the sand and clay bluffs and by the 

 presence of fixed dunes, the western ends of which have been entirely 

 eroded away by lake action. 



2. Nipissing formations. Beyond Point Betsie extending to the 

 northeast from the western end of the northern glacial ridge is an ex- 

 panse of sand ridges of slight elevation with depressions between. This 

 formation extends six miles to Platte River, and beyond that toward 

 Empire, and is about one mile in width. It apparently dates from 

 Nipissing times, and is very similar to the characteristic formation of 

 similar origin at the head of Lake Michigan in Indiana and Illinois. As 



