TOPOGRAPHY AND GEOLOGY. 13 
These rocks are, for the most part, nearly horizontal, but show gentle 
folds at some places, while at others they are somewhat faulted. 
Ordovician rocks appear also in an area south of the Arkansas 
River Valley, in the Ouachita Mountain country. In this region they 
include the novaculites and the rocks immediately underlying them. 
They are exposed in portions of Pulaski, Saline, Garland, Hot Spring, 
Montgomery, Polk, and Pike counties. a 
The Ouachita Ordovician rocks are composed chiefly of novaculites 
and shales. They have been pressed together from the north and 
south, so that the beds have been thrown into a series of sharp east- 
west folds. After they were folded erosion cut away the tops of the 
arches, leaving the hard novaculites as prominent ridges over the 
whole area. 
SILURIAN SYSTEM. 
Silurian rocks are known only in the northern part of the State. 
They rest conformably on beds of Ordovician age, but have been 
recognized at only a few places — in the region just west of Batesville 
and at points as far west as St. Joe, in Searcy County. These 
Silurian beds are limestones, at some places overlying a bed of shale 
of probable Ordovician age. The areas of these beds are too small 
to be represented on the accompanying map (PI. I), so they have 
been included with the Ordovician. 
DEVONIAN SYSTEM. 
The Devonian period is represented by beds of variable character — 
in some places a black shale, in others a sandstone containing phos- 
phate nodules. The thickness of the shale ranges from a few inches 
to 70 feet; that of the sandstone ranges from 5 to 75 feet. These 
rocks are known only in northern Arkansas, where they follow the 
contact between the Silurian and Carboniferous rocks from near 
Batesville westward to the vicinity of St. Joe, in Searcy County. 
They have been recognized also along White River northeast of 
Fayetteville, and in the northwest corner of the State, at Sulphur 
Springs, where both shale and sandstone are well developed. Although 
the phosphatic nodules occur sparingly in places in the lower few 
inches of the overlying basal Mississippian limestones, there was prob- 
ably a break in sedimentation between the two formations, and the 
nodules have been reworked into the younger beds from the debris 
of the older. This formation also is so limited that it is mapped with 
the Ordovician rocks. 
«A report on this area by L. S. Griswold appears in Ann. Rept. Geol. Survey Arkansas for 1890, 
vol. 3, 1891. 
