Geology of the High Falls Dam Site, Part 6: Rectangular Jointing and a Mysterious Circle | Marinette County, Wisconsin, USA

Facing northwestward and looking at another exposure of glacially abraded Belongia Granite. For more on the significance of this pink- to red-tinted component of the Mesoproterozoic Wolf River Batholith, see Part 5.

This shot may be the murkiest of this particular set of old High Falls slide transfers, but it still shows two items of special interest.

In earlier Flickr photo posts of such differing locales as Sardinia, Upstate New York, and the Chisos Mountains of western Texas, I've pointed out various examples of fractures in rock known as joints.

It's thought that most joint patterns found in outcrops are the result of stone that's gradually been freed of confining pressure by the process of erosion. After millions of years of being buried by overlying units of rock, the recently exposed stone stretches vertically but shrinks horizontally—a process known as the Poisson Effect—and so fractures.

Rectangularly disposed joint patterns are particularly common, and that's what we have here. The individual joints are nicely delineated by mosses enjoying the moisture and nutrients that accumulate in them.

But there's also another type of depression in the granite, and a perfectly circular one. It has nothing to do with the Poisson Effect, but rather with what I call the Pinhead Syndrome. It's the sorry remains of a benchmark installed by surveyors of one of a number of government agencies, most notably the Geodetic Survey and the US Geological Survey.

Benchmarks are brass or bronze disks inscribed with the site's exact elevation and other important information. It's a violation of federal law to damage or remove them.

So of course there's a sizeable class of human collectors who delight in prying them up with crowbars, pocketing them, and displaying them next to their horde of stolen license plates. These are the same sort of folks whose main food groups are grain alcohol and boiled woodchuck.

You'll find the other photos and descriptions of this series in my Natural History: Wisconsin album.

Taken in: United States / Wisconsin / Stephenson (show map)

Taken on: May 10, 2002

Tags:

geology

nature

rocks

stone

Wisconsin

High Falls Dam

Marinette County

Belongia Granite

outcrops

glacial abrasion

jointing

benchmarks

granite

Wolf River Batholith « less

Sets appears in: • Natural History: Wisconsin • Rock Forms, Rock Formations

Groups appears in: • Wisconsin • Rocks and Geology more » • Geology Rockers • Midwest • Granite • GeoTagging Flickr • Geotagged • Rocks • Geology • Rocks and Stones • Geologists • Wisconsin Nature Photography • USA - UNITED STATES OF AMERICA • nature and landscape photography • North American Nature & Wildlife Photography • Geology for all • American Photography • Rocks-Fossils-Minerals • Nature Photography // • stones n rocks • I Love Rocks! • Midwest Nature • Natural Midwest *Please invite friends* • Geotagged: Wisconsin • Geology Flickrs • Nature Album • Photography with a Story • Geology / Geologie • Nature and all that is beautiful • The Very Great Outdoors • We love Nature and Geology • AMERICAN PHOTOGRAPHER « less

Camera: MAKER NAME 96660, ISO 50 (more info) (hide info)

Camera:	MAKER NAME 96660
ISO:	50
Flash:	Auto, Did not fire
Exposure Program:	Program AE
Metering Mode:	Average
White Balance:	Auto
Color Space:	Uncalibrated
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