324 PROCEEDINGS OF THE ACADEMY OF [1893. 



that the streams on the Cretaceou3 have been rejuvenated by the late 

 Tertiary and Quaternary elevation in the first of which the area 

 under consideration was elevated above sea level as a young 

 unscored plain ; secondly, that the streams on the Tertiary are 

 chiefly extended, hence well developed lower portions of streams 

 and their development is more rapid than the headwater streams 

 which are also higher above sea level and hence have a greater 

 depth to cut through before reaching base level ; and thirdly, 

 the fact that the Tertiary strata are unconsolidated while those of 

 the Cretaceous are in large part indurated. Topographic age is a 

 relative term varying according to circumstances, not the least 

 important of which is the character of the strata. In soft strata 

 topographic maturity will be reached much sooner than in hard 

 strata, other conditions being equal. 



(c) Origin of the Flood Plains. — The broad flood plains of the 

 forested area are worthy of some attention. In the spring and early 

 summer such streams as the Brazos in this region overflow their 

 banks and transform the swampy malarial plains upon either side 

 into impassable lakes, very much as does the Red River of Arkan- 

 sas. The immediate cause of this is the excessive floods which come 

 from the rains at the headwaters. The reason why such excessive 

 flooding is possible is that the rivers of Texas are overburdened 

 streams. The sands and clays of the Permian and Carboniferous 

 areas cause the streams which flow through them to be overbur- 

 dened far up toward the headwaters, and this condition continues 

 and increases through the Cretaceous and Tertiary areas. In the 

 forested region, the slope being diminished as the sea is approached, 

 much of the sediment burden can be carried no farther. This ten- 

 dency to deposit the sediment in the stream beds has been exagger- 

 ated by the recent slight submergence indicated by the fjords and 

 estuaries at the river mouths in the Quaternary plain. In this way 

 the deposits near the mouths of the streams have been brought 

 about in the effort to restore the profile of equilibrium, and secure a 

 channel-way to the sea through which the sediment load can be 

 transported. This establishment of the profile of equilibrium is now 

 in progress and the flood plains are growing up stream. 



For some reason there has been a change in conditions of recent 

 date. Formerly the rivers flowing through this Tertiary area were 

 able to keep their channels clear and carve them deeper. Now they 

 are building them up, and this in spite of an elevation of some fifty 



