Part of Southern Indiana (the Mitchell Plain) is classified by
geologist as a karst region. Karst regions are characterized by
caves, sinkholes, sinking rivers and underground drainage systems.
The term karst comes from an area of Yugoslavia where these
characteristics have been extensively studied. In the U.S., karst
landscape may be best studied from southern Indiana through
Kentucky and into Tennessee.
Formation of karst features depend on solution of underlying
soluble rocks. Abundant rainfall in this area provides the water
for this, but water alone will not create these features. The water
must be slightly acidic. This occurs by solution of an atmospheric
gas, carbon dioxide, in the water to form carbonic acid. The
soluble rock is limestone, which is made of the mineral calcite
which readily dissolves in carbonic acid. These areas of the United
States are underlain by vast areas of limestone. One more condition
that is required is that there is a way for the acidic water to get
into the layers of limestone. The limestone must have joints, or
fractures, for the water to seep through. The St. Louis and St.
Genevieve formations under the Mitchell plane are ideal. As the
water seeps through the many joints and along bedding planes of
these formations, the limestone is dissolved away leaving a cavity
(opening). Over time, the cavity enlarges and eventually becomes a
cave which may contain an underground stream system (such as the
Donaldson/Twin Caves) which is fed through sinkholes, another karst
feature formed as the acidic water works its way down through the
joints.
In the case of Twin Caves, a couple of sinkholes caused the
collapse of the cave ceiling exposing the underground stream. This
is called a karst “window”, and it allows us to see a
short segment of the underground stream. This stream continues for
a short distance as subsurface drainage and then emerges from the
mouth of nearby Donaldson Cave and becomes surface drainage.
A short boat ride into one of the caves is available for a small
fee on weekends April to Memorial Day and Labor Day to the end of
October. Trips are daily from Memorial Day weekend to Labor Day.
Participants on these tours have an excellent chance of seeing
bats, blind crayfish, and the endangered Northern blind cave
fish.
To log this cache, you MUST submit a picture from the
coordinates, showing the cave that has the boat rides and yourself
with your GPSr at the time you submit the log. In addition, to meet
new logging requirements, you must determine an approximate flow
rate for the water. This can be done by timing the movement of an
object floating on the water (twig or leaf) over a predetermined
distance and calculating the flow rate in feet per minute. Email
your answer to me at time of logging. Logs submitted without flow
rate submitted by email WILL BE DELETED by owner. There is a gate
fee to enter the park. When you stop at the gate, be sure to get
maps and brochures of the park. There are many interesting things
to see and do in the park.
This earthcache has been approved by NSS.