LOW MOUNDS OF THE PRAIRIES. 241 



less than 4 feet above the general level, with a profile approximately of 

 turtle-back shape. In distribution they are most abundant over the lower- 

 lying and more level areas. They are generally not arranged in any special 

 order, but lie scattered promiscuously at varying intervals. At times, how- 

 ever, they are seen in rows, which arrangement makes them much more con- 

 spicuous and heightens the artificiality of their appearance. The material 

 of these mounds seems uniform throughout and is essentially the same as the 

 surrounding soil, though of somewhat superior fertility. It is opener, lighter 

 and apparently richer in organic matter. Here the grass grows most lux- 

 uriantly and clumps of bushes or small trees are often found. Neither in 

 the arrangement of the material nor in the contents of these mounds is there 

 evidence of the agency of man in explanation of their origin. The mere 

 fact of their great number would preclude this. They stand simply as pro- 

 ducts of a gentle erosive action in soft homogeneous material. Their sur- 

 faces represent a former general level. Their material is the old top soil. 

 The intervening depressions are caused by the slow soaking and solvent 

 action of rain accompanied by a gentle flow. 



Prairies are scattered over nearly the entire area illustrated on the map, 

 and an enumeration of them all would be impracticable here. Notable in- 

 stances are : French prairie, at the western terminus of the Washburn anti- 

 cline ; Long prairie, north of the Backbone anticline ; Massard prairie, 

 south of Fort Smith ; Potato hill prairie, south of Charleston ; Grand 

 prairie, north of Charleston ; and the prairies east of Russellville. 



The River System. — The river system of this area is a theme worthy of a 

 separate chapter. Its evolution and the adjustment of the courses of the 

 different streams will not be discussed here, as this would expand the paper 

 beyond reasonable limits. A few points of interest in this connection may, 

 however, be referred to. 



As already stated, this region may be considered as having attained the 

 stage of topographic maturity. The Arkansas river has about reached its 

 base-level ; the flow is even and not swift ; there are no falls nor bars nor 

 rapids over rocky bottoms, and a layer of sand or silt generally intervenes 

 between the water and the underlying rock ; corrasion has about ceased and 

 the stream is extending its alluvial plains laterally. Of a previous stage of 

 base-leveling there is more than a suggestion in the approximate uniformity 

 in height of the majority of those elevations which we have termed ridges 

 and mesas. To fully realize this, however, a careful study of the detailed topo- 

 graphic sheets is necessary. 



The rocky gorge through which the Arkansas flows, gives it, in places, the 

 character of an antecedent channel. This is especially noticeable at a point 

 about thirty miles below Fort Smith, where the bluffs on either side for a dis- 

 tance of nearly ten miles come down to the water's edge almost continuously. 



XXXVI— Bull. Geol. Soc. Am., Vol. 2, 1890. 



