248 QUICKSILVER DEPOSITS OF THE PACIFIC SLOPE. 



This valley must once have been much deeper than now and in part have 

 silted up. 



Burns Valley, an area near the town of Lowei' Lake, the whole of 

 Big Valley, and portions of the country about Upper Lake, as well as 

 many small flats along the lake shore, are clearly also covered with recent 

 lake deposits. 



The causes which might have produced this shrinkage of the surface 

 of the lake are erosion of the outlet or orographical changes, or both. Had 

 the lake bed only been tilted to the southeast, the tendency would have 

 been to expose the bottom to a considerable depth at the west end, but not 

 near Lower Lake. Orographical changes alone are consequently insuffi- 

 cient to explain the exposure of the meadows. Cache Creek, just beyond 

 the limits of the map, passes through a narrow gorge of Cretaceous sand- 

 stone, and the mere erosion of this barrier would produce the effect under 

 discussion, but there is some evidence to show that orographical causes 

 have influenced the grade of Cache Creek and consequently its capacity 

 for eroding its bed. On the north fork of Cache Creek the banks are ex- 

 tensively terraced and four or five flood plains are distinctly visible. This 

 appears to mean a tilting of the country to the eastward, though probably 

 to the extent of oidy a very few inches to the mile. Such a widespread 

 secular change would increase the velocity of Cache Creek, as well as of 

 its northern fork, and accelerate the erosion of the sandstone gorge near 

 its source. Were the circumstances more favorable, such a change, if it 

 really took place, might be detected on the banks of the lake, which would 

 also be terraced. Two causes seem to have stood in the way of such a 

 modification of the shore. The first of these is the resistance offered by 

 the sandstone of the gorge, which would yield but slowly to an increased 

 velocity of a current carrying little sand in suspension. It is well known 

 that the erosive power of lake water is slight because it is so free from 

 sand, and the erosion of the goi'ge would probably be still slower than it is 

 were it not for the fact that Herndon Creek flows into Cache Creek between 

 the lake and the gorge. The second influence tending to prevent the for- 

 mation of terraces is the tule belt. A strip of these reeds-, from a few feet 

 to a few yards In width, grows almost everywhere along the lake shore. 



