18 THE CLAYS OF ARKANSAS. 
OCCURRENCES PRECEDING THE CRETACEOUS. 
Deposition of the rocks from the Ordovician beds to those of the 
Pennsylvanian took place over the same general area. The expo- 
sures of the Mississippian and the rocks below that horizon have 
resulted from the erosion or removal of rocks that once overlay them. 
Had the entire area of the State continued submerged during Meso- 
zoic time, triassic, Jurassic, and Cretaceous rocks would most prob- 
ably be found to overlie the Carboniferous formations. The distri- 
bution of the Cretaceous rocks, however, shows that at the close of 
Carboniferous time a large area became dry land, and that the shore 
of the Cretaceous ocean crossed the southwestern part of the State 
somewhere below the northern border of the Cretaceous area shown 
on the geologic map (PI. I). 
CRETACEOUS SYSTEM. 
Rocks of Cretaceous age occur in but a small portion of the 
State. They are present, concealed in some places by later deposits, 
in a roughly triangular area south of the Ouachita Mountains and 
west of Arkadelphia. The Tertiary-Cretaceous border lies slightly 
southeast of the St. Louis, Iron Mountain and Southern Railway 
between Arkadelphia and Texarkana. The rocks are not folded or 
bent, but dip to the southeast at a low angle. They are usually uncon- 
solidated, contain considerable calcium carbonate, and form, on decay, 
a very fertile black soil. The Cretaceous deposits have been described 
in detail, a so that they need not be further mentioned here, except 
to call attention to the fact that later observations have shown that 
the Arkadelphia shale at Arkadelphia is Cretaceous and not Tertiary, 
as was originally supposed. Cretaceous rocks have also been recog- 
nized at a few places north of Little Rock. 
TERTIARY SYSTEM. & 
Clays, sands, and gravels of Tertiary age cover the greater part of 
eastern and southern Arkansas. In the rocks of this age and near 
their base, at Benton and near Malvern, there are valuable beds of 
potter's clays. These beds dip gently to the southeast. They are 
all more or less sandy, and but few of them are hard and consolidated. 
At the lignite mines of Ouachita County, however, some of the sands 
are indurated to very compact sandstones, and at some places in 
Crowleys Ridge they form the hardest of quartzites. Though the 
a Hill, R. T., Ann. Rept. Geol. Survey Arkansas for 1888, vol. 2, pp. 66-174; also Taff, J. A., The chalk 
of southwestern Arkansas: Twenty-second Ann. Rept. U. S. Geol. Survey, pt. 3, 1901, pp. 687-742; 
Branner, J. C, The cement materials of southwest Arkansas: Trans. Am. Inst. Min. Eng., vol. 27, 
1898, pp. 42-63. 
b For details of the Tertiary geology of the State see The Tertiary geology of southern Arkansas, by 
G_. D. Harris; Ann. Rept. Geol. Survey Arkansas for 1892, vol. 2, 1894, 
