Caves of the County (continued) -

In a state known for its cheese, it may not come as much
of a surprise that geologists, hydrologists and cavers draw an
analogy to what is under foot. The Door Peninsula bedrock
is so full of holes that it is often likened to Swiss cheese.
The rocks are so full of holes that those outcropping along
the Niagara Escarpment on the west side of the peninsula
sometimes whistle when the wind blows.

Caves are the underground natural plumbing of the
county, and they come in all sizes. Some, such as Horseshoe
Bay Cave or Paradise Pit Cave, are large enough to walk or
crawl through. Most holes in the rock, though, are too small
to fit people, but plenty big for ground water flow. Caves,
open joints and fractures connect the surface water with the
ground water systems in the county. Sinkholes can be seen
throughout the county and represent surface depressions
created when a cave ceiling collapses and reaches the
surface. This process forms the natural entrances to many
caves. It also means that what you pour into a sinkhole may
show up in your sink later that day!

There are three types of caves in the Door Peninsula:
solution caves, the so-called “sea caves,” and fault-generated
caves. All form along the dominantly northeast and
southeast oriented joint structures that have developed in
the Niagara Dolomite.

The solution caves are the most common. One of the
best examples is Horseshoe Bay Cave south of Egg Harbor.
The cave is privately owned and its entrance is steel gated,
but cavers such as Bob Bultman from the Wisconsin
Speleological Society and other lucky visitors on occasion
get to crawl under the county through more than a halfmile
of passages in what may be Wisconsin’s longest cave.
Horseshoe Bay Cave is wet and muddy, so much so that
one section is called the Mississippi River. Portions of the
cave in fact do represent an underground river, and
sometimes during spring runoff the ground water flows in
torrents and issues forth from the mouth of the cave. The
cave shows features of this ground water flow through the
water sculpted walls and pothole-like geometry of some of
the rooms. The rooms and galleries in the cave also
display modest dripstone cave formations, such as stalactites,
stalagmites and other delicate features. These mostly exist in
the deepest portions of the cave, where they survive visitors
or the onslaught of spring runoff and sediment. Many of
the solution caves were formed before the great ice age that
descended upon Northeast Wisconsin over the past million
years. The melting of the glaciers flushed uncountable tons
of sediment into the caves, and has all but filled them up
with mud, silt, sand and gravel.

Sea caves, a misnomer since they exist along the modern
and ancient shorelines of Lake Michigan and Green Bay, are
shallow caves formed by wave action that slowly dissolved
the dolomite along open joints and fractures. These caves
are common along the Niagara Escarpment and can be
easily seen at Cave Point Park south of Jacksonport. These
caves are distinctive in their lack of dripstone formations,
short passage extent, and their occurrence on cliff faces.
They were formed during the last glacial epoch between
10,000 and 100,000 years ago.

The third type of Door Peninsula cave was formed
through fault activity. These caves are not common and
the best example of one is the Dorchester Cave in Sturgeon
Bay. The cave was likely formed during an earthquake
that rattled the peninsula and caused one of the structural
joints to shift and open to a width of three to five feet. This
action created a linear cave that was discovered during the
excavation of the basement for the Dorchester Retirement
Home. Gary Soule, an experienced caver who has visited
virtually every cave in the United States, recognized the
importance of this discovery in 1973 and convinced the
Dorchester owner, John Penny, to preserve the cave. Soule,
with his own money, mapped the cave and worked with
the project’s contractors to install two steel doors in the
concrete foundation of the Dorchester. The first door
accesses the northern extension of the cave, and the second
allows entrance to the southern extension. The cave is
mostly filled with glacial outwash sediment and rock debris
but is an excellent example of the structural control of caves.
flat joint surfaces of the cave’s walls have been subjected to
slow dissolution following the earthquake, and this has revealed
beautiful fossils in the dolomite.

Door County’s caves are literally buried gems in the peninsula. Sculpted by groundwater and now mostly buried by glacial sediment, scientists are still learning a lot about the peninsula’s underground world. Some of the caves are accessible but none have yet been developed into commercial enterprises. This means the public can only visit them through
special arrangements with property owners. But, the next time you notice a depression filled with rocks in a farm field, consider that it is a doorway to the caves beneath the Door Peninsula.