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University of California. 



LVOL. 2. 



of the Siestan formation, and upon them in turn the massive flows 

 of the later basalts. The removal of the latter, except at the head 

 of the valley, is due to the gradual sapping of the soft underlying 

 beds, causing the formation of an escarpment, or line of cliffs, which 

 has receded by the undermining process. That recession has 

 gotten as far as the tip of the spoon, and the process is there still 

 active. The sapping action is facilitated by the fact that the upper 

 beds of the Siestan formation, being chiefly of clay, are impervious 

 to water, so that the waters which percolate through the overlying 

 basalts, the latter being jointed and fissured, come out at the base 

 of the basalt in the form of springs in the synclinal axis. These 

 springs aid materially in washing away the clay, and the basalt, 

 thus robbed of its support, crumbles down under the action of 

 gravity. When the front of the escarpment was farther down the 

 valley, the springs were of course more copious in their flow than 

 they are now, with the present limited catchment area of the 

 surface of the basalt, so that the sapping process is less rapid than 

 it formerly was. 



Stratigraphic Asymetry of Syncline. — Having in mind the some- 

 what conventional ideas of geological structure that are acquired 

 from reading and from text-book diagrams, we might expect, hav- 

 ing recognized the general synclinal structure revealed by the ero- 

 sion of Siesta Valley, that the two limbs of the syncline would be 

 practically symmetrical. This, however, is far from being the case. 

 Sedimentary deposits are essentially lenticular in their form and 

 frequently thin out rapidly. It is owing to this characteristic of 

 the beds deposited in the Siestan Lake that the northeastern limb 

 of the syncline differs in its petrographical composition from the 

 southwestern limb. The limestone formation, which was barely 

 detected in a line of outcropping blocks as the basal bed of the 

 Siestan formation on the southwestern limb, is here present, but in 

 much stronger development, reposing directly upon the underlying 

 volcanics. In addition to this, however, there is a second more 

 important limestone bed fifty or sixty feet higher up in the forma- 

 tion. This limestone is from ten to twenty feet in thickness, and is 

 well exposed at several places in the trench of the creek which 

 drains the east fork of Siesta Valley. 



