Lost River - Karst Information Portal

This short paper, presented here in its
entirety, while somewhat a summary of pre-
1976 publications, also has a few very interesting
figures, notably Figures 10 and 11) which depict
interpretations from much hard map and field
work, as well as a hint to explorationists of where
to Tlook for Mitchell Plain caves.
he Mitchell Plain of southern Indiana is
a sparsely dissected, low plateau in which
much of the surface has been subjected to karst
development. It is bounded on the west by the
Crawford Upland, an area of ridges capped by
the predominantly clastic Chesterian series,
and on the east by the Scottsburg Lowland, a
broad plain formed on shale. Sub-accordant
summit elevations of the Crawford Upland
at altitudes of 900 to 1,000 feet are believed
to represent the hypothetical Lexington-
Highland Rim peneplain (Thornbury, 1965, p
191). The Mitchell Plain, which lies at altitudes
of 700 to 1,000 feet, is a composite, low relief
plateau formed by a combination of erosional
and depositional events. Analysis of the
geomorphic history of the area is complicated
by the low gradient of the plateau surface,
which nearly coincides with the regional dip
of the rocks (about 30 ft/mi) into the Illinois
structural basin. Despite local evidence for
structural control, however, the entire surface
is subtly discordant to the regional dip, in that
the land surface possesses a gentler slope.
The Mitchell Plain is made up of three
landform types and contains evidence for at least
three different erosional-depositional surfaces
transecting the various landforms, The three
landform types include: (1) areas of sinkhole
plain developed along the down-dip edge of the
Mitchell Plain in the vicinity of the entrenched
rivers, at relatively low elevations; (2) areas
mantled with residuum and unconsolidated
sediment to the east of the sinkhole plain,
farther removed from the entrenched rivers;
2
The Mitchell Plain of Southern Indiana
By Margaret Palmer
(1976 NSS Bulletin, Vol 4, “Indiana Karst Symposium” slightly edited by Strunk, 2007)
and (3) finely dissected areas of non-karsted
limestone and shale ridges, at relatively high
elevations in the furthest up-dip parts of the
Mitchell Plain (commonly considered to be
the western portions of the Norman Upland,
but included here as part of the Mitchell Plain).
Evidence for the three erosional-depositional
surfaces that have been developed across the
landforms can be established from (regional)
cross sections (see Figures 9 and 10). Tracing
the surfaces is difficult, as later dissection has
unevenly lowered much of the landscape.
The different erosion-depositional surfaces
include: (1) the upper Mitchell Plain surface, best
preserved on the gently-sloping upper surfaces of
the residuum and unconsolidated sediment, but
also represented by the highest divides between
sinkholes in the sinkhole plain and by relatively
flat ridge tops in the dissected region; (2) the
lower Mitchell Plain surface, which represents the
lower limit of unconsolidated cover, preserved
as low-elevation saddles between sinkholes on
the sinkhole plain, and (3) the Blue River Strath
(Powell, 1964) which is represented by karsted
valleys heading in the sinkhole plain. A slight
break in slope between the Blue River Strath
and the sinkhole plain proper suggests that
the Blue River Strath is actually a later surface
entrenched below the Mitchell Plain. Although
drainage is predominantly in the direction of
the (westerly) dip, the different surfaces transect
the geologic structure. The upper Mitchell Plain
surface is formed on the varied lithologies of the
Ste. Genevieve, St. Louis, Salem, Harrodsburg,
and Borden units.
Because erosion, solution, and deposition
were all active during their formation, the
surfaces can best be described as “base-leveled
surfaces.” A more detailed discussion of the
geomorphic history of the Mitchell Plain can
be found in Palmer and Palmer (1975). The
long periods of rather static base level necessary