92 GEOLOGICAL HISTOEY OP LAKE LAIIONTAN. 



The conditions most favorable for the formation of barrier bars obtain 

 when shelving shores occur adjacent to steep banks where sea-cliffs are 

 forming; in such instances the debris derived from the sapping of the sea- 

 cliff is swept along by shore currents, and furnishes the material for works 

 of construction. 



Sometimes a current is deflected from the shore and returns to it at 

 another point. In this manner a looped bar inclosing a lagoon is formed. 

 The lakeward portion of such a bar sometimes forms a definite angle; the 

 structure then becomes V-shaped, and is known as V-bar. Bars with this 

 peculiar form are not uncommon, and sometimes obtain great magnitude. 

 It frequently appears as if structures of this nature had been begun in a 

 rising lake, and that the forms of the shore deposits first made were 

 retained and carried upwards as the lake rose, by the addition of fresh 

 material to their surfaces. Barrier bars present other variations, some of 

 which will be noted in the succeeding pages, which may frequently be 

 seen in process of formation on the shores of existing lakes. 



Bars of another character are also formed along lake margins, at some 

 distance from the land, which agree in many ways with true barrier bars, 

 but differ in being composed of homogeneous, fine inaterial, usually sand, 

 and in not reaching the lake surface. 



The character of structures of this nature may be studied about the 

 shores of Lake Michigan, where they can be traced continuously for hun- 

 dreds of miles. There are usually two, but occasionally three, distinct sand 

 ridges; the first being about 200 feet from the land, the second 75 or 100 

 feet beyond the first, and the third, when present, about as far from the 

 second as the second is from the first. Soundings on these ridges show 

 that the first has about 8 feet of Avater over it, and the second usually 

 about 12; between, the depth is from 10 to 14 feet. From many com- 

 manding points, as the summit of Sleeping Bear Bluff, for example, these 

 submerged ridges may be traced distinctly for many miles. They follow 

 all the main curves of the shore, without changing their character or having 

 their continuity broken. They occur in bays as well as about the bases 

 of promontories, and are always composed of clean homogeneous sand, 



