334 PROCEEDINGS OF THE ACADEMY OP [1893. 



times several hundred feet in length. Where the creek flows 

 through a flood plain of gravel the channel is between gravel 

 banks, generally less than thirty-five feet wide and twenty-five feet 

 deep. At flood time not only is the channel filled, but the sur- 

 rounding plain is covered with water for some distance, and during 

 these times the erosive work of the year is done. 



(c) Effect of the Superimposition of the Colorado on the Silurian. 



(i) Temporary Base Level. — The Colorado River and its tributa- 

 ries which drain this region, are superimposed through the Creta- 

 ceous upon the buried Palaeozoic. It so happens that the Colorado 

 itself has found, in its down cutting, a hard spur of Silurian marble 

 and it is now wrestling with this obstacle. Over this spur the cur- 

 rent flows with numerous rapids, and the stream is here very busy 

 deepening its channel ; the precipitous walls of its canon, which 

 begin here, show that the work has for some time been in progress. 

 The softer Carboniferous rocks upstream from this, yield much more 

 readily and the stream above the Silurian is waiting for the pro- 

 gress of the work below. The river is eroding on the Silurian but 

 is building up on the Carboniferous for a score of miles above the 

 natural dam, since the river is always heavily burdened. 



This accident to the river has given rise to several interesting 

 peculiarities, not only the river itself but all its large tributaries being 

 influenced by the retardation. They have reached a temporary 

 base level, their mouths are widening and extensive flood plains are 

 being built. This is seen in the Pecan Bayou and in the San Saba 

 River. The latter in its upper course flows in a deep canon in the 

 Silurian where it is still at work eroding, but for eight or ten miles 

 from its mouth it has reached the temporary base-level and is not 

 at work eroding its channel. 



The flood plain of the Colorado is frequently fifty feet deep and 

 is being rapidly built up. This is proved by buried trees, both dead 

 stumps and partly buried living pecans. In flood-time great quan- 

 tities of sediment are brought down from the headwaters and 

 deposited here and during the winter very little is done toward 

 removing this material, for even then the river is heavily burdened. 



In a river of such habits as the Colorado the valley is, as would 

 be expected, peculiar. The ordinary channel, though quite wide 

 and deep, is incapable of holding even the ordinary floods. Such 

 floods, which may occur several times in a year and last for several 

 days, do not rise to the level of the flood plain proper, but 



