K. Bryan — Rock Tanks and Charcos. 201 



able drainage area lies above the falls. This upland 

 valley has relatively gentle slopes, and the grade of the 

 stream through it is in marked contrast to the grade in 

 the falls section, which is approximately 1 to 1. 



The rock of this part of the mountains is a coarse, 

 pegmatitic granite. Most of the surfaces are strained 

 brown from the deposition of limonite, especially in the 

 cove of Tinajas Altas. In such places there are many 

 caves, reentrants, and pinnacles which add much to the 

 picturesqueness of the mountain slopes. 



The details of the falls, and the form and location of 

 the plunge pools, are largely affected by the joints of the 

 granite. The master joints strike N 2° E, and dip 65° 

 NE, and divide the granite into great slabs, varying in 

 thickness from 2 to 10 feet. A less perfect system has a 

 strike N 80° E, and an almost vertical dip. 



The profile shown in fig. 9 is made from the plane 

 table location of the tanks, and an aneroid determination 

 of the top of the falls by C. G. Puffer. It shows that in 

 general the falls are parallel to the dip of the joints, and 

 the plunge pools are located at places where the joints are 

 closely spaced. In plan, also, the course of the stream 

 is controlled by the joints, as shown in fig. 10. The 

 steepest grades are parallel to the minor joint system, 

 and the flattest grades to the master joint system. The 

 plunge pools are located on the steps developed at the 

 intersections of the joints. 



The lower tank (No. 1 in fig. 8) fills with sand after a 

 flood, and therefore the water lasts longer than in the 

 others. The second tank is accessible by climbing along 

 a cable fastened to the rock about 75 feet south of the 

 channel. From the second tank the third is easily 

 reached. The fourth and higher tanks are very inac- 

 cessible, and but little used except by mountain sheep. 

 Animals, however, frequently meet disaster by falling 

 into the water and drowning. Two dead sheep were 

 seen in the upper tanks on October 22, 1917. 



Sand Tanks. 



Sand tanks are a variety of rock tanks formed in 

 stream channels in the various ways previously outlined, 

 and differing from other rock tanks only in being filled 

 with sand. The tail end of the flood carries sufficient 

 sand to fill the cavity, whereas with ordinary rock tanks 

 the latter part of the flood is relatively clear. The sand 

 thus deposited is saturated with water. The upper por- 



