POST-TERTIARY DEPOSITS. 59 



UPLAND GRAVEL. 



The Upland Gravel occurs in the shape of pebbles capping even the 

 highest hills in the region, and is especially well represented on the summits 

 of the high bluffs of the larger rivers. On the Brazos, Colorado, and Rio 

 Grande it forms beds from one to fifteen feet and more in thickness. Back 

 from the rivers, however, as far as has been seen by the writer, the gravel 

 becomes much less plentiful, and often occurs simply as scattered pebbles, 

 overlying the eroded surface of the Tertiary sands and clays. Hill* speaks 

 of great beds of gravel, "Plateau Gravel," in Southwest Arkansas, which 

 probably represent the "Orange Sands " of Hilgard. This Red River region 

 has not been visited by the writer, but to the south of it the gravels, except 

 nearer the larger river courses, occupy a most subordinate place in the 

 topography of the country. 



According to Hilgard, the "Orange Sands" occur in great abundance in 

 Mississippi on the tops of the high hills and ridges, but in the part of Texas 

 in question the hill tops are capped by soils resulting from the immediately 

 underlying Tertiary strata, and the only representatives of the gravel are a 

 few scattered pebbles. If we suppose the water courses at the time of the 

 deposition of the gravel to have run in the same general direction as now, 

 it is natural to expect a greater development of the gravel beds in the region 

 of the Mississippi embayment than in Texas, as that river, carrying more 

 water and having an influence over a much larger area than any Texas river, 

 not only had a larger region to draw gravel from, but also had more power 

 to transport it, and a larger region to spread it over. Hence the much 

 smaller development of these deposits in Texas than in Mississippi. These 

 pebbles in Texas decrease in size and quantity as we go down stream, and 

 vary cousiderably in their composition on the different rivers. This is to be 

 expected, as the rivers rise in the different regions, and hence their sources 

 of supply of pebbles are different. On the Colorado they consist of lime- 

 stone, flint, quartz, silicified wood, jasper, and rarely granite and feldspar. 

 The limestone and flint have come from the Cretaceous rocks and are fre- 

 quently fossiliferous. The silicified wood is rounded and worn, and has 

 doubtless come from the Tertiary strata of the region. The other pebbles 

 are from the area of crystalline rock in Burnet and Llano counties. The 

 scarcity of granite and feldspar is due to the ease with which they decompose, 

 and consequently the only representative of the granite that is usually seen 

 is the quartz. 



On the Brazos the pebbles consist of fossiliferous limestone, water worn 



* R. T. Hill, Arkansas Geological Survey, Vol. II, 1888, p. 29. 



