The Origin and Geology of Caves

THE ORIGIN AND GEOLOGY OF CAVES

Woodward Cave is a solutional cave, formed by the removal of bedrock by circulating groundwater and by underground streams.  The water transports the bulk of the material out of the cave in solution (i.e., - dissolved).  Most solution caves are in limestone.

Karst Topography

Areas with abundant limestone caverns have karst topography.  The term karst describes a landscape of characteristic landforms and subsurface features produced primarily as a result of solution of the underlying bedrock.  The solution process is accompanied by other processes, particularly slumping of soil and bedrock materials, transport of insoluble debris through subterranean routes, and some mechanical wasting of slopes.

The most characteristic landform is the sinkhole or doline.  Surface draining is often underground so that sinking streams, underdrained valleys, and big limestone springs are the common features.

Cave Features

Tunnels - Caves are made of passages.  The passages have various shapes, various relationships to each other, may have formed at the same or at different times, may have been formed by a single water source or by different sources.  Passages in a cave may be genetically related or only fortuitously connected.

Passage cross-sections vary from elliptical tubes formed from fast-moving water to more complex shapes controlled by the characteristics of the bedrock.

Vertical Features - A remarkable aspect of most caves in regions of moderate-to-low relief is that the principal passages are either nearly horizontal or almost vertical.  The vertical features may be conveniently divided into two types:  chimneys and vertical shafts.

Chimneys are the common type of vertical opening in Pennsylvania caves and typically are solutionally widened joints or fissures, or solution passages extending along bedding planes in near-vertical limestone.

Passage Terminations - If cave passages were formed by flowing water, why would there be a sudden end to a tunnel?  The tunnels end (truncated), when rocks collapse (breakdown).

Pennsylvania caves are usually highly truncated since they occur under limestone valleys at shallow depths.  Instead of miles of interlacing passages and tunnels, typical Pennsylvania caves have lengths of hundreds of feet.  Often a single fragment is all that is accessible.  Some are terminated by breakdown, but many more terminate in clay chokes.  The caves all contain layers of silt and clay on the floors, and in places these fill deposits reach the ceilings and effectively block the passage, although presumably it was open to groundwater flow at some earlier time.

Entrances - To the explorer, the entrance is the most important feature of the cave.  Without it, the cave goes forever undiscovered.  The entrance is not an intrinsic part of the cavern development process.  The cave forms without an entrance.  The entrances are developed later as the down-wasting of the landscape dissects and truncates the cave system.

Cave Rooms - The usual room is a place in which the cave widens or heightens above the average passage through which the explorers have been traveling.  The origins could be:

• Intersection of several passages.
• Places where breakdown has fallen while groundwater was actively circulating.
• A fragment of large conduit reached through smaller conduits.
• Vertical solution intersected by small horizontal passages.
• Actual widening of horizontal passages.

Speleothems - The familiar calcite formations that decorate most caves are known as speleothems.

The most common formations arise from water dripping or flowing in the cave, giving rise to the general terms of dripstone and flowstone.

Water emerging from joints in cave ceilings hangs there in drops for a short time before the drops fall to the floor.   During the time in which the drop hangs, a small amount of mineral matter is deposited in a ring with a diameter similar to that of the drop.  The ring grows downward at constant diameter as more material is deposited until a slender tube of calcite known as a soda straw stalactite is formed.  Eventually, minerals deposit on the outside of the straw, the central canal may become clogged, and the formation evolves from the primary tube to a pendant form.

Stalactites fed by more than one drip point may grow into quite complex shapes.  Water flowing over the outside builds up ribs and folds to yield the form known as the drapery.  Water that trails along the underside of ledges may build up a sort of unfolded stalactite in which the growth layers are linear and parallel to the ledge and which is known as the bacon strip.

Water dripping to the floor of the cave deposits more minerals and builds up the mass of travertine known as stalagmites.  Many are fed by more than one drip and take on complex shapes.

Solutions flowing down walls and over ledges deposit masses of travertine with the appearance of a waterfall of rock.  These deposits are called flowstone and can be of very large volume.

Other type of speleothems are created more from crystal growth than dripping water.   These include helictites, botryoids, and gypsum flowers.

Helictites are stalactite forms that grow in curved paths, often upward, instead of hanging vertically.  Botryoidal forms are small bead or know-like projections from cave walls.  Gypsum flowers consist of crystals of gypsum growing outward from a common center.