36 THE CLAYS OF ARKANSAS. 
which there are but few valuable beds of shale. In the great river 
bottoms of the Arkansas there are large tracts covered by river silts 
that are not available for the manufacture even of the most ordinary 
building bricks. However, there is not a single county in the entire 
Paleozoic area in which fairly good brick clays may not be found. 
CRETACEOUS AND TERTIARY CLAYS. 
It is only for the sake of convenience in presentation that the Cre- 
taceous and Tertiary clays of the State are here treated in the same 
chapter. A glance at the geologic map of Arkansas accompanying 
this report (PL I) shows that rocks of Cretaceous age outcrop over a 
relatively small part of the State." They are found in only about 
seven counties. 
During Tertiary time, a period not far removed from the present, 
geologically, a large part of the State of Arkansas was covered by 
the ocean. The western shore of the Tertiary sea entered the State 
on the southwest somewhere in the vicinity of Ultima Thule, and 
passed into Missouri near the line between Clay and Fulton coun- 
ties. The western margin of the Tertiary, as shown on the geo- 
logic map (PL I), marks approximately the old shore line. With the 
northern limits of this sea we are not now concerned, but on the east 
it extended nearly to Tennessee River, covering all of eastern Arkan- 
sas, while almost all the State of Mississippi lay beneath its waters. 
All waters that flowed into this Tertiary sea carried down great 
quantities of sands and clays, just as to-day sands and clays are 
being carried out into the Gulf of Mexico by the Mississippi and by 
all the surface waters that flow into it. This material was spread 
out over the floor of the sea in beds as nearly horizontal as the nature 
of the bottom and the currents would permit, and with them were 
mingled the shells and bones of animals that lived and died in the 
water and the leaves and stems of plants that were floated out by 
the streams. Later the land was elevated until the silt of that ancient 
sea bottom now stands 375 feet (at Little Rock) above the present 
ocean's level. 
During this depression of the land the Tertiary clay beds of the 
State were deposited, and the conditions under which these beds 
were formed satisfactorily account for their characters, their distri- 
bution, and their included fossils. 
Should we go back a little further in geologic time we should be 
able to account also for the clay beds of the Cretaceous region in 
the southwestern corner of the State. The conditions under which 
they were deposited differed but little from those under which the 
Tertiary clays were formed. Neither the Cretaceous nor the Tertiary 
/a The Cretaceous geology of the State has been described by Prof. R. T. Hill in Ann. Rept. Geol. 
Survey Arkansas for 1888, vol. 2. 
