62 A GEOLOGICAL SIJKVEY OF LANDS 



to within less than two niiles of the Del Rio road, where it turns 

 first to the south and then to the east, following more or less 

 closely to the north boundary of block 4. 



The low lands on the north side of this block are mostly 

 underlaid by this formation. In block 5 only the larger streams, 

 the two branches of the Sycamore, have cait through the lime- 

 stone cover into the clay, but in block 11 the formations have 

 been lifted a little higher and erosion has laid bare the Devil's 

 river limestone everywhere in the north tier of sections, and in 

 block 10, while extensive low flats mark the outcrop of the cla^^ 

 south of this for a varying distance of from one to three miles. 

 Following these flats to the east we find them turning southeast 

 toward Pinto mountain and then east along Pinto creek, until 

 we reach the head of Grass valley. From here the outcrop of 

 the Del Rio clay runs nearly south for about five miles and 

 then east, making a short detour to the south around Turkey 

 mountain. 



Relation of the Del Rio Clay to Water Supply. 



This clay is practically impervious to water. We therefore 

 find water accumulating under it as well as above it. On the belt 

 of lowlands just described, and for two or three miles south of 

 them, many wells draw w^ter from the limestones underneath 

 this formation, the best water-bearing ledges being reached about 

 a hundred feet below its base. These are exposed to view in the 

 bed of Evans creek, in survey 19 in block A, west of Devils river. 

 All the large springs of this region, such as the Cienega spring 

 northwest of Del Rio, the San Pedro spring, the Las Moros 

 springs, and even the Brackett spring, are evidently overflows over 

 or near the north margin of this clay, which limits the deep under- 

 ground seepage under the country to the south, and has caused 

 the ground waters to flow over or to find outlets and establish 

 open passages through the less well protected margin of this con- 

 fining cover. 



Accumulation of water on the upper surface of the clay has 

 resulted in the making of a number of waterholes on the belt of 

 flat lands over its area of outcrop. Such are the Javelin water- 

 hole in survey 16 in block 11, and many other less important 

 ones, more or less permanent. 



