90 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



BARS. 



In the illustrations given above the effects of veaves that result from 

 •winds blowing directly on-shore are alone considered. When the wind blows 

 obliquely to the beach we have another modification of wave action. Cur- 

 rents are established in each instance by the friction of the wind on the 

 water, but in the first they are at right angles to the beach and return lake- 

 wards as an undertow; in the second case, however, i. e., when the wind 

 blows obliquely to the land, the currents formed move more or less nearly 

 parallel with the shore, as may be illustrated along any lake margin by 

 placing floats in the water or by watching the movements of the shore-drift. 

 It will require but little attention to assure one that during a gale strong 

 currents are thus established along lake margins, which in some respects 

 are similar to the flow of rivers. They carry with them a band of shore- 

 drift, consisting of sand, gravel, bowlders, etc., the width of which depends 

 mainly on the slope of the shore and the character of the material moved. 

 As in the flow of streams, the transported material is carried partly in sus- 

 pension and partly by rolling along the bottom. The upward wave move- 

 ments tend to lift the stones and the onward movement to carry them for- 

 ward. When the force of the current is checked the coarser debris is first 

 deposited and the finer transported to greater distances. The movement of 

 such current-borne streams of c?e6ns along a lake shore necessitates friction, 

 which results in the comminution . of the debris itself and the abrasion of 

 the base of the sea-cliff' against which it impinges. Shore currents are even 

 more powerful than on-shore waves as agents of erosion, but their distinc- 

 tive property is the power to transport shore-drift. In this manner the 

 debris supplied by the sapping of sea-cliffs is removed and formed into new 

 structures at the same time that it causes the detachment of fresh material 

 from the shore, thus supplying fresh tools with the aid of which the waves 

 remodel their boundaries. 



Shore currents are usually strongest at some distance from the actual 

 lake marg-in. In some instances this distance amounts to several rods or 

 perhajDS half a mile. As the maximum transportation takes place where the 

 current is most rapid, the result is the formation of a ridge of gravel in the 

 path of the current. Deposits of this nature are molded by the waves 



