Cenozoic Era Formations
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Fort
Union formation
(Tour
Map symbol 'Fu')
Tertiary period strata was deposited as bands from erosional and accretional
events and ash from volcanic activity. Sandstones in the Ludlow
and Cannonball members of the Fort Union formation (statiographic
map) form the resistant caps of the buttes in northwestern South
Dakota. Sandstone concretions of
all sizes have weathered from the Cannonball member and are actively sought
by collectors. The sandstones of the Tongue River member form large
cliffs within which many caves are
found.
White
River formation
(Tour
Map symbol 'W')
In south-central South Dakota, the strata of the White River formation form
the rugged terrain of the Badlands
National Park between Wall and Interior. These sediments are terrestrial
in origin and were derived from erosion and accretion events. They also contain
layers of ash from numerous volcanic events. The Brule member consists of light
multicolored siltstones and forms the sharp ridges that overlie less resistant
clays and mudstones of the multi-banded Chadron member. Weathered
Chadron sediments are sometimes called haystacks because of their rounded
appearance.
In northwestern South Dakota, there is a small exposure of the White River formation located in the Slim Buttes of eastern Harding county. The Brule formation is overlain by strata from the Arikaree forming spectacular eroded formations and outcrops.
