TOPOGRAPHY AND GEOLOGY. 
17 
The soil derived from the rocks of the "Lower Coal Measures," 
being deficient in lime and in some places rather thin, is only moder- 
ately fertile. 
Hartshome sandstone. — The Hartshorne sandstone lies at the base 
of the productive coal-bearing rocks of the Arkansas coal field. It 
is known to have a great areal extent, and is found cropping out 
around the edges of the coal-bearing rocks from the east end of the 
Arkansas coal field westward into Oklahoma. It is 100 to 300 feet 
thick, and contains minor beds of shale in its central and upper parts. 
McAlester group. — Above the Hartshorne sandstone there is in the 
productive coal-bearing rocks a series of shales and sandstones with 
a number of beds of workable coal. The McAlester group is divisible 
into three formations — (1) a lower, known as the Spadra shale, con- 
sisting of three or more beds of coal and minor strata of sandstone; 
(2) a middle, called the Fort Smith formation, composed chiefly of 
sandstone and shaly sandstone beds with one or more workable beds 
of coal; (3) an upper, described as the Paris shale, consisting partly 
of beds of sandy shale with some sandstone and one or more work- 
able beds of coal. These formations of the McAlester group are 
Fig. 1.— Ideal section across folds in the Carboniferous rocks of western Arkansas, sh, Shale; 
ss, sandstone. 
described in the report on the Arkansas coal field. a The Spadra 
shale is 400 to 500 feet thick, the Fort Smith formation 375 to 425 
feet, and the Paris shale 600 to 700 feet. 
Savanna formation. — Overlying the McAlester group there is in the 
productive coal series a formation consisting of several sandstone 
members separated by shales. This is known as the Savanna for- 
mation. It occurs in Arkansas only in the tops and upper slopes of 
Poteau, Sugarloaf, Short, and Magazine mountains. That part of 
the Savanna exposed in Arkansas is estimated not to exceed 1,000 
feet, and constitutes approximately the lower two-thirds of the 
formation. 
The rocks of this formation, as well as the other rocks of the produc- 
tive coal series, are all more or less folded, so that the shale outcrops 
depend on the character and direction of these folds and can there- 
fore be determined only after a study of the structure of the region. 
It can be said, however, that the shale outcrops generally lie in the 
valleys parallel to the ridges, so that a cross section of the region 
would show the rocks to have the relations shown in fig. 1. 
a Bull. U. S. Geol. Survey No. 326, 1907. 
48136— Bull. 351—08 2 
