Shades State Park: A Missing Part of the Geologic Story (GM-33)
Trail I, following a niche high on the canyon wall, passes just beneath a
bluff of sandstone. Rocks exposed here reveal an irregular surface of
contact between sandstone above and siltstone below. The sandstone was
formed during the Pennsylvanian Period about 235 million years ago; the
siltstone, Mississippian in age, is about 250 million years old. The
surface between them, called an unconformity, represents a gap of about 15
million years in geologic time. It tells of a period of erosion separating
the deposition of the rocks above and below. In a few places fragments
torn from the older rocks are found where they were redeposited among the
younger sediments above.
In other places rocks that fill this gap in the
geologic record are present. But at the Shades, it is as if these pages
have been torn from the story of the past.
Computer graphics oblique aerial view
Our Hoosier State Beneath Us: