“Ship Rock,” an isolated pinnacle of Cambrian sandstone

July 13, 2011


While in the Air Force, I worked with many naval officers at the Pacific Command in Hawaii. I recall how they would always laugh about being a “lonely ship at sea.” On Hwy 21 in Adams County, Wisconsin, not far from Coloma, stands something known as “Ship Rock,” which the University of Wisconsin-Green Bay has dubbed “an isolated pinnacle of Cambrian sandstone.” The professionals there also call it “one of the easternmost of the castellated mounds of central Wisconsin.” She’s a lonely ship on the plain.

If you are in a hurry and whizzing down Hwy 21, like I was, and you see this for the first time, it comes as a total surprise and you have to postpone what you were doing and stop to take a look. That’s what I did.

Regrettably, vandals have covered some of the formation with graffiti, but you learn to look past that to study the rocks.

Roche-A-Cri is French and means “crevice in rock.”


Well, okay, so what do we have here? A “castellated mound.” The term “mound” refers to an isolated hill. UW Green Bay tells us this:

“The term is most commonly applied to the castellated mounds, isolated hills of Cambrian sandstone rising steeply above the central lowlands, and occasionally capped by Ordovician dolomite. Usually, they simply consist of sandstone, and are often steep-sided pinnacles. They are far too delicate to have survived glaciation, and many owe their steepness to wave erosion by Glacial Lake Wisconsin. They are ephemeral features and will be gone in a few tens to hundreds of thousands of years.”

Cambrian sandstone is usually white or gray. The Cambrian period of the Earth’s development occurred 542-488 million years ago. A castellated mound is one which has grooves. Ordovician is the period that followed the Cambrian period, roughy 488-444 million years ago. Dolomite is carbonate mineral composed of calcium magnesium carbonate.


This map was presented by UW-Green Bay and was redrawn from Lawrence Martin’s 1932
The Physical Geology of Wisconsin. The black arrow points to “SH,” which is the code he used for Ship Rock. Each red dot is a mound. So if you want to go on a mound hunt, you can see the geographic area in which to look.

So let’s take some looks at this Ship Rock.


This big guy actually stands just to the east side to the main formation, quite close to it.

I learned from watching the kids visiting that there is a trail around the rock, so I took it.






I keep trying to remind myself that I once was a kid who knew no fear. It so happened that I found at least one with his buddy.


Well, there they are. The guy on top trying to convince the guy below that he knows what he is doing. Let’s zoom in on these two.


I would soon learn that the guy with the red shorts on feels he has found a tunnel through the rock, I think. You can see the opening right where is left hand is located.


So in he went. You can see the red and black soles of his sneaker Now I do not know what the other guys is doing, either saying I want no part of this, or going to the other side to see if his pal emerges. A zoomer on the lad’s feet.


I did some photoshopping which produces an awful photo, but I can tell you after I brighten this up as much as I could, and you can sort of make it out here, his legs are heading downward!

I did not stick around to see what happened. I’ve got enough problems!

Just a few more geological notes on the formation. UW-Green Bay has done some considerable work studying these rocks. In one of its reports about a field trip taken in May 2008, I noted that Professor John A. Luczaj “wondered if siliceous fluids hadn’t percolated along the joint and made the rock more resistant to erosion. Dr. Luczaj is Associate Professor of Geoscience, Chair of Geoscience, Department of Natural and Applied Sciences at UW-Green Bay. “Silicieous” is the adjective of a hard, colorless compound that occurs as the mineral quartz and as a principal constituent of sandstone.

Much of this section of the state is generally flat and sandy, described by some as a lake plain. They apply that description because their analysis reveals that Ship Rock stands on what was the bottom of Glacial Lake Wisconsin. The web site
“Geocoaching” has said this:

"This lake did not last long, about 15,000-19,000 years ago, at the end of the last Wisconsin ice age. The lake was located in the "central sand plains" ecoregion of the state. This portion of the state is generally a flat, sandy lake plain which was formed in and around Glacial Lake Wisconsin. The soil here is primarily sandy lake deposits.

"Sandstone buttes carved by rapid drainage of the glacial lake, or by wave action when they existed as islands in the lake, are distinctive features of this landscape."