EMBANKMENTS NEAR THE CARSON DESERT. 113 



cliffs, and depositing it in less exposed situations under the lee of the prom- 

 ontoiy where tlie shore receded and formed a bay. The sickle-shaped bars 

 with free extremities, which resulted from this action, indicate by their forms, 

 as well as by the character of the pebbles and stones composing- them, the 

 direction followed by the currents to which they owe their origin. The hill 

 represented at the top of the accompanying map is of white rhyolite, with a 

 high, narrow spur of black anamesite projecting from its southern border. 

 The latter rock also composes the hills represented in the southwestern por- 

 tion of the map. Between these rugged outcrops there is a gentle slope of 

 alluvium, cut by miniature drainage lines. The striking contrast in the color 

 of the rock in place at either end of the embankments enables one to deter- 

 mine at a glance the localities from which the stones composing them were 

 derived. 



The broad, lightly shaded bands on the right of the map, having a 

 southeast bearing, are low gravel bars that form the crest of the divide in 

 the pass, or ancient strait, and are now separated by a smooth playa. The 

 drainage north of these bars is into South Carson Lake, while the rill-lines 

 south of them combine to form a water- course that leads eastward past 

 Allen's Springs into the desert valley which opens southward. The curved 

 bars, shown in the central portion of the map, are thus confined in vertical 

 range between the highest water-line of the former lake, i. e., the Lahontan 

 beach, and the highest part of the pass in which they occur. This interval 

 measures 114 feet, or, in other words, the water was 114 feet deep in the 

 shallowest part of the strait at the time the highest embankment was formed. 



The highest of tliis series of p-ravel structures are shown at A and 

 B on the map. A is a short, curved bar, 3 or 4 feet higher than the 

 long, sickle-shaped embankment on which it rests, and is composed of Avell- 

 worn stones and gravel of anamesite and rhyolite. That this bar was built 

 subsequent to the much longer embankment beneath is proven by the 

 stratification to be observed at its terminus. Its outer or lakeward 

 slope is regular, with a curved contour that is the result of deposition. 

 Had the lower embankment been built last, the outer slope of the higher 

 structure would have been cut away by shore erosion, so as to form 

 a sea-cliff. The embankment at B is at the same horizon as the smaller 

 MoN. XI 8 



