360 li. S. Tarr — Supcrimposition of Drainage in Texas. 



particularly noticeable in valleys carved in soft clays and shale. 

 For instance, the Walclrip Division of soft coal-bearing beds, 

 for a distance of more than 80 miles, is marked by a topo- 

 graphic depression. A single stream does not follow this line 

 of weakness continually as would very likely have been the 

 case had the Carboniferous formed an original drainage-sur- 

 face, but small streams have their head-waters here and larger 

 creeks flow in it for some distance before leaving it to cross 

 the hard underlying limestone. 



Everywhere may be seen signs of attempts at rejuvenation, 

 but it was not until lately that I was able to see that the Colo- 

 rado itself shares this peculiarity- This river flows with a very 

 serpentine course through the Carboniferous, having a length 

 along the boundary of San Saba county of 50 miles, while at 

 the end it is only 30 miles from the first point. In one place 

 it makes a bend six miles long where a cut off would reduce 

 the distance to two miles. Several possible reasons suggest 

 themselves in explanation of this phenomenon which is quite 

 remarkable in a river with a fall of from two to three feet per 

 mile. Since the Colorado is a superimposed river flowing in 

 its present course chiefly by accident, it is possible that before 

 the Quaternary uplift the river may have been sufficiently old, 

 in this part of its valley, to have the serpentine course common 

 to such rivers. Thus the present form of valley may be in- 

 herited from that time. Another possible cause is that the 

 slow removal of the Silurian by retarding the down-cutting in 

 this part of the river-channel has induced a temporary old-age 

 condition. That this is the case to a certain degree is abund- 

 antly proved by the extensive flood plains of the Colorado and 

 its side streams ; but whether this is a sufficient cause to ac- 

 count for the phenomenon at present under consideration is 

 doubtful. It may be that both these causes have had some 

 effect, but the chief cause is quite different and is to be found 

 in the futile attempt of the Colorado to adjust itself to the new 

 conditions which it has found in its enforced path, probably 

 aided somewhat by the Silurian barrier which has prevented 

 rapid down-cutting. 



The evidence of this attempt at readjustment is that all the 

 main bends in the river have one, and generally both, of the 

 long sides of the loop parallel to the strike of the Carbonifer- 

 ous. This is the case in eleven distinct instances, and in one 

 case the river flows northeast for four miles before turning and 

 cutting across the strike to resume its natural course. Above 

 Elliot Creek in Mills county there is a stratum of coarse, thick- 

 bedded sandstone, which has deflected the Colorado river south- 

 west along the strike for a distance of three and one-half miles 



