204 



by hand and carted beyond the reach of the high storm waves, and after- 

 wards loaded and shipped by rail to Chicago, where it is nsed in roofing 

 and concrete pavements. The immediate source of this gravel doubtless is 

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

 since the composition of the pel>bles plainly show that they came originally 

 from formations which lie far to the northwest."' 



If a person stands uiwn the southern shore of the lake and observes 

 the waves coming in, he will notice that each wave carries up a small 

 quantity of sand, and when it is rolled up far enough to be out of the 

 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 whicli the wind blows for a considerable length of time is generally 

 marked with ripples just as the sand in the bottom of a shallow 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 blows over the surface 

 the current is turned upward, and as it passes over the crest an e<ldy is 

 left on the leeward side and the grains roll over the crest and droj) 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 the velocity be doubled 

 the carrying power is sixty-four times as great. Consequently any increase 

 in velocity rapidly increases tlie carrying and erosive power. 



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

 of the shores are angular pieces of quartz, 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 appearance. 



Vegetation. — The .surface of a great part of the dune area is without 

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

 bare, while in the swales between the ridges are shrubs and grasses of 

 distinctly sand soil types. The bare surfaces gleam in the siuilight 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 are practically covered with black and barren oaks, north- 



ilndlana State Geological Report, 1S07, p. 41, Blatehley. 



