TITLES AND ABSTRACTS OF PAPERS 67 



The gravels referred to are of Pleistocene age and are remnants of terraces 

 now undergoing destruction by weathering and erosion. 



Origin of the Gravels 



Gravel of the same kind is now in the process of making in the wide chan- 

 nels of several of those tributaries to the river which drain the Edwards 

 plateau. Las Moras and Tequesquite creeks when flooded roll streams several 

 hundred feet wide of such bouldery gravel. The average size of the boulders, 

 or pebbles, is perhaps 2 inches in diameter. The limestone from which these 

 boulders are worn is usually thick bedded and lacks for the most part marked 

 laminated structure. The texture of the limestone is superficially somewhat 

 uniform in all directions. Cleavage is about as easy in one direction as in 

 another. The result is that the pebbles and boulders of this stream gravel are 

 spheroidal in form. They weaf round. A certain small per cent of pebbles 

 of flattened form can indeed also be found. This consists of limestone, with 

 well defined laminated structure and consequent parallel planes of easy 

 cleavage. 



There can be no doubt that the remnants of terraces referred to were built 

 up from similar gravels transported into the Rio Grande and rolled by this 

 river in the same manner. On the American side of the river the gravels are 

 most abundant for a distance of 20 miles below where the river leaves the 

 Edwards plateau, between Del Rio and Eagle Pass. 



Description of different Kinds of Flattening 



On the upper level tracts of these terraces and on their dissected slopes it 

 is often difficult to find a pebble or boulder of limestone that is spherical. All 

 are flattened, oblate spheroids. Looking at their shapes more closely, we find 

 now and then a pebble in which the longest diameters all around mark a 

 sharpened edge. It is evident that such edges never were worn to their pres- 

 ent shape by trituration in a gravel stream. Some pebbles are hemispherical 

 in form, having one side more flat than the other. On the pebbles of most 

 pronouncedly flattened form one frequently finds shallow concavities on the 

 surface, showing the result of etching by solution. In very rare cases one 

 may see etched grooves radiating from the elevated center of one of the flat- 

 tened surfaces of these boulders. This radiated, furrowed sculpture is one of 

 the most common sculpture forms seen on the bare limestone surfaces in situ 

 in this part of America. 



Conclusions 



It is quite evident that the flattening of these small boulders is caused by 

 the solvent action of the rain water. It is often greatest on the exposed upper 

 surface of each boulder. But the process is clearly so slow that most! of the 

 boulders have had time to be turned over, perhaps many times, by the creep- 

 ing of the upper surfaces of the terrace and by other superficial accidents due 

 to the wind, to plants and to animals tramping and burrowing into the ground. 

 With the smallest beginning of flattening there will always be a tendency of 

 a boulder to lodge whenever disturbed, with its equatorial plane in a hori- 

 zontal position. The hemispherical boulders were probably subjected to pro- 

 longed planation in one undisturbed position. The ultimate result of the 



