108 GEOLOGICAL HISTORY OF LAKE LAHONTAJf. 



crosses it, but maintains a nearly horizontal crest for at least a third of the 

 way to the point where the river has cut through ; it then falls off with an 

 abrupt descent to a level six or eight feet lower, the crest of the structure 

 at the same time becoming broadened and curved slightly westward. Con- 

 tinuing southward, one descends three more similar scarps of less height 

 before reaching the lowest point in the embankment. Each of these 

 descents is formed by the end of a comparatively thin layer of gravel that 

 was added by the currents to the surface of the structure, and would no 

 doubt have been carried along its whole extent had not a rise in the lake 

 caused the currents to begin the formation of another similar sheet of gravel 

 near the shore and at a higher level. Each of the steps in the crest of tlie 

 embankment represents a pause in the rise of the waters of the ancient lake. 

 The highest in the series was the last formed. The incompleteness in tliis 

 instance furnishes the suggestion that similar embankments which seem from 

 their foi'ra to be homogeneous may in reality be highly compound. The 

 irregular stratification of the embankment retaining Humboldt Lake is 

 illustrated by the following sketch of the section exposed on the right side 

 of the channel that has been eroded through it The general inclination of 

 the strata on the west side of the embankment is much greater than on 



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Feot 



VsrliccJ and Horizontal ScaJe 

 FlQ. 17.- Section of ffr.lvel embauknient at wost end of Humboldt Lake. 



the east, the reason being that in deposits of this nature the scari)s sloping 

 with the current are usually steeper than those inclined in the direction 

 from which the current arrives. The lines of unconformability sloping 

 gently westward in the upper part of tlie section indicate periods of erosion 

 when the top of the structure was removed and subsequently rebuilt with 

 current-bedded gravels. At the time these alterations were made, the lake 

 surface in each instance must have been nearly on a level with the top of 

 the embankment. 



The embankment crossing the valley is older than the branching 

 structures forming the surface at either end, as is shown by the superposi- 

 tion of the latter. This is well exhibited in the left side of the gap cut by 



