204 



liy liaiid aiul carted beyond the reach of the high storm waves, and after- 

 wards loaded and shipped hy rail to Chicago, wliere it is used in roofiug 

 and concrete pavements. Tlie inmiediate source of tills gravel doubtless is 

 the blue glacial till which forms the greater part of the floor of the lake, 

 since the coiiipdsitinn of the pehlilcs jilaiiily show that they <-aiiii' originally 

 from formations which lie far to the northwest."^ 



If a per.son stands upon the southern shoi'e of the lake and observes 

 the waves coming in, he will notice that each wave cai-rics up a small 

 (jnantity of sand, and when it is rolled up far enough to be out of th(' 

 reach of other waves until it has had time to dry it is rolled farther inland 

 by the wind and is added to the great mass of sand already accumulated, 

 which goes to build up the dunes and the ridges. The surface of dry sand 

 over which the wind blows for a considerable length of time is generally 

 marked with ripples just as the sand in the buttom of a slinllow stream 

 The ripples are small, but their shape and structure is the same as that 

 of the larger dune of which they are a part. The long gentle slope of the 

 dune is formed on the windward side. As the wind bl()ws over tlu> surface 

 the current is turned upward, and as it passes over the crest an eddy is 

 left on the leeward side and the grains roll over the crest and dro]) down- 

 ward. Objects in the path of the dune influence the outline of the dune 

 as shown in the page of diagrams. The transporting power of the wind 

 varies as the sixth power of the velocity, i. e., if tlie velocity be doubled 

 the carrying power is sixty-four times as great. Conse(iucntly any increase 

 in velocity rapidly increases the carrying and erosive power. 



The grains of sand freshly brought up from the lake from the erosion 

 of the shores are angular pieces of (piartz. but soon become rounded by 

 abrasion. The sand of the Lake Michigan region is of a light brown color, 

 but when viewed at a distance in the sunlight has a very white apjiearance. 



Vegetation. — The surface of a great part of the dune .area is wilhoui 

 vegetation. The tops and sides exposed to the winds arc in most cases 

 bare, while in the swales between the ridges are shrubs and grass(>s of 

 distinctly sand soil types. The bare surfaces gleam in the sunlight and 

 give the appearance of great snowdrifts. On cloudy days the top of the 

 ridges, the clouds and the lake in the background present a confused out- 

 line. Farther inland the vegetation gets a better hold on the sand and 

 many of the hills arc imictically covered with black and barren oaks, north 



'Indiana State Oeolojileal Ro])ort. 1,sn7, p. 11. Blatcliloy. 



