lawson i 



I'AI.ACHK J 



The Berkeley Hills. 



3*7 



To the southeast from our line of section the terrace loses its 

 character as such and passes rapidly into the floor of Siesta Valley. 

 The lower limit of the Siestan formation determines closely the 

 southwestern edge of the valley bottom, the steep upper slopes or 

 walls of the valley being the dip slope of the underlying lavas. 

 This condition continues for some miles beyond the limits of our 

 immediate field. 



Synclinal Trough. — When, now, we turn to trace out the line of 

 demarcation between the Siestan formation and the overlying 

 basalts in the same southeasterly direction, we find that it contin- 

 ues parallel to the lower limit of the formation for only a short 

 distance. About half a mile from our line of section the trace of 

 the plane of contact between the two formations makes a sharp 

 turn around the base of a bluff of basalt, and then bears away to 

 the north, along the west side of the east fork of Siesta Valley. In 

 this vicinity there are numerous excellent exposures of the Siestan 

 beds, and from an examination of these we see that this sudden 

 change in the direction of the contact line is accompanied by an 

 equally sudden reversal in the direction of the dip. 



The Siestan beds dip under the basalt as before but in a west- 

 erly direction. From these observations it becomes apparent that 

 both the Siestan formation and the overlying basalts are folded in 

 the form of a trough, and that the bluff around the base of which 

 we have traced the outcrop of the plane of contact is the tip of a 

 spoon-shaped area of basalt reposing on the Siestan beds. Below 

 the tip of the spoon, Siesta Valley broadens, and its floor is under- 

 lain only by the Siestan formation, except for a line of heaps of 

 basalt blocks extending down the middle of the valley, which are 

 the residua of the former extension of the overlying basalt. 



On the northeast side of Siesta Valley below the tip of our 

 basalt spoon the Siestan beds repose, with uniform southwesterly 

 dips upon the surface of the volcanic rocks which form the precip- 

 itous wall of the valley on this side. We thus learn that Siesta 

 Valley is a syncline, and that the geomorphy of the valley is deter- 

 mined by that fact. On either side of the valley are massive vol- 

 canic flows dipping towards the center line of the valley. This 

 center line is the axis of the syncline. Upon these lie the soft beds 



