IN THE UPPEK mo GKANDE EMBAYMENT IN TEXAS. 69 



here it is continued as a narrow belt under the north cliff of the 

 Anacacho mountains. 



Physical Properties. 



These clays weather rapidly and their detritus is promptly 

 carried away by erosion. As a result we find the lands they un- 

 derlie, flat and low and with a fine and fertile soil. The draining 

 streams have cut wide and open valleys. Their channels have in 

 several places been gouged out by the freshets to hollows which 

 are deep enough to hold water the 3'ear around. They are quite 

 as impervious to water as is the Del Rio clay and afford every- 

 where suitable ground for the construction of tanks. On the 

 other hand these clays yield no water in wells. The streaks of 

 sand which they contain, are of small extent, and at best give 

 only a small amount of seep water. 



Thickness. 



Owing to the scarcity of outcrops and fossils the thickness 

 of the Upson clay can only be made out from the general dip of 

 the formations and from well records. The mantle of soil is thin, 

 but covers the formation everywhere, except at a few points in 

 the arroyos, a.nd one may travel for miles without seeing any- 

 thing below this mantle. Fossils are likewise scarce and no 

 attempt could be made to combine the exposures seen into a 

 single section. Of the thickness of this formation I can hence 

 only make an estimate based on the general dip of the bed rock 

 for this region, which appears not to exceed seventy feet per mile 

 along the Rio Grande. As the width of the belt of outcrop is about 

 seven miles, this indicates a thickness of nearly 500 feet. In block 

 7 several wells have been bored in this clay, but the records are 

 notofsucha nature that it is clear whether they havegonethrough 

 the formation or not. It is evident, nevertheless that there are 

 some four or five hundred feet of clay in the country south and 

 east of Spofford. 



At all events the thickness of the Upson clay is not the same 

 for different parts of this region. Approaching the Anacacho 

 mountains it thins rapidly. Its upper part is here replaced by 

 the Anacacho limestone and only the basal part continues as a 

 rapidly eroding stratum in the foot of the Anacacho escarpment 

 for a limited distance. 



Library Publications. 5. 



