96 GEOLOGICAL UlSTORY OF LAKE LAtlONTAN. 



iently designated as J -bars. Other modifications of gravel-built structures 

 will be noticed in the descriptions of the shores of Lake Lahontan which 

 follow. 



Water at rest having no power to erode, it is evident that lakes modify 

 their shores but little during calm weather ; it is when storms are rap-ing 

 that the potency of waves and currents reaches a maximum, and the greater 

 part of terrace cutting and bar building takes place. The level of the high- 

 est water line as recorded by works of construction, is the storm level ; in 

 some instances this is several feet above the normal lake surface, for the rea- 

 son that the water is raised to an abnormal height along a shore against which 

 a gale is blowing. The topography of a coast may cause the storm waves to 

 reach a higher level in some portions than in others, as, for example, whercs 

 a funnel-shaped bay opens out into a broad lake. In such an instance the 

 water will be driven into the bay during on-shore storms and forced to a 

 greater height than on a more open coast. For these reasons the highest 

 beach-lines of a lake at various points are not always in the same plane ; ;i 

 fact that should be borne in mind while measuring the depth of fossil lakes 

 and in studying the effects of orographic movement. 



DELTAS. 



The general forms of the fan-shaped accumulations of gravel, sand, and 

 silt deposited about the mouths of streams which enter still water, are too 

 well known to require a detailed description. 



When a stream bringing silt and sand in suspension and rolling peb- 

 bles and larger rock masses along its bed debouches into still water, its 

 momentum is checked and the greater part of its load is deposited. When 

 the structure thvis begun is vindisturbed by currents it is built out eqiialh' 

 in all directions from tlie mouth of the stream and thus acquires a semi • 

 circular or fan-shaped topographic form. When a high-grade stream enterii 

 a valley it commonly deposits a heap of debris about the point of discharge, 

 which has received the name of an alluvial cone; a delta may be consid 

 ered as an alluvial cone that has been formed with its base below water, 

 The part of a delta that is above the reach of the waves has the irregular 

 structure characteristic of alluvial deposits, but the submerged portion 



