FIELD TRIP TO HOCKING HILLS
Saturday, 6 November 2010 saw the geology club take a
field trip to southeastern Ohio, to the scenic Hocking Hills (Hocking Hills
State Park). First on the itinerary was Old Man’s Cave and its
associated gorge and waterfall. After lunch, the group headed to Rock
House.
Old
Man’s Cave area
Near-vertical, vertical, and even overhanging
sandstone cliffs occur along a stream gorge in the Old Man’s Cave
area. The main geologic scenery is the Black Hand Sandstone.
Old Man’s Cave
Gorge just upstream from Old Man’s Cave.
The dominant direction of creek erosion is downward. The rocks making up
the walls of the gorge are relatively resistant, planar-bedded to cross-bedded
quartzose sandstones, pebbly sandstones, and conglomeratic sandstones of the Black
Hand Sandstone. The Black Hand Sandstone is ~mid-Early Mississippian
in age (late Kinderhookian or early Osagean Stages). It is traditionally
interpreted as a delta deposit, but in recent years has been shown to be an
ancient incised valley fill deposit.
Old Man’s Cave - the overhanging sandstone cliff that the group is
looking at is Old Man’s Cave itself. The rocks are part of the
Black Hand Sandstone.
Waterfall a little downstream from Old Man’s Cave. This visit was
during a time of low flow in the stream, so the waterfall was a mere trickle.
Rock
House area
Differential weathering and erosion of jointed Black
Hand Sandstone has resulted in a “cave” along a cliff face called
Rock House. The only chamber extends parallel to the exterior cliff
face Several side openings (keyholes entrances) are present that connect
the chamber with the cliff face.
OSUN Geology Club at Rock House
Rock House - the “cave” known as Rock House is behind & parallel
to this cliff face, to the left of the viewer. The rocks are dominated by
planar-bedded and cross-bedded quartzose sandstones of the Black Hand
Sandstone, an ancient incised valley fill deposit of ~mid-Early
Mississippian age.
Rock House - one of several entrances to the chamber. Rock House is not a
true cave. Nowhere in the cave can one get beyond twilight (= one of the
criteria for true caves). Most caves are the result of partial
dissolution of limestones. The bedrock here is principally quartzose
sandstone (Black Hand Sandstone). The greenish coloration is due to the
presence of living algae on the surface of the rocks. The red coloration
is due to hematite
staining (Fe2O3).
Rock House - the long axis of this chamber (~along the viewing direction) is
developed along a major joint surface (easily seen in the ceiling) in the Black
Hand Sandstone. Differential erosion has significantly enlarged the joint
plane, resulting in a “cave” big enough to walk through.
Several side openings (along the right wall as one view this photo) are
developed along joint surfaces roughly perpendicular to the main chamber
fracture.
Rock House has been recently cited as an Earth
analogue to a type of cave that may be present on Mars. If life evolved
on Mars, and if that life still exists, it would have to be in the subsurface,
most likely in cryptic environments or “refugia” such as caves.
Mars has lava tube caves (collapsed and non-collapsed) as well as overhang
caves and crevice caves. Rock House in the Hocking Hills of Ohio has been
mentioned by planetary geologists as the kind of cave that future astronauts
might explore on Mars in search of life.
Honeycomb Weathering - many Black Hand Sandstone outcrop surfaces have
well-developed honeycomb weathering, formed by small-scale differential
weathering and erosion. The shaded, subvertical feature on the left side
of the photo is one of numerous joints (fractures) in the Black Hand
Ss. The greenish & pale coloration on the rock surface is from the
presence of living algae and lichen.