Geology of the High Falls Dam Site, Part 7: What's the Point? | Marinette County, Wisconsin, USA

Facing north-northwestward.

The garb worn by this returning tour group is ample evidence that in northern Wisconsin even the latter half of May can be pretty chilly and blustery. The Paper Birches haven't fully leafed out yet, but at least there's one sign of spring: some Rose Family trees (Canada Plums? Serviceberries?) are in full bloom in the background.

But, getting back to this post's interrogatory title, what is the Point? Well, it's the name that's been given to this rocky, glacially abraded mini-promontory that sticks out like a sore thumb into the musky-infested waters of the High Falls Reservoir. The nearer highline tower stands at its northern tip.

We've come only about 200 yd (184 m) from the mass of jointed Belongia Granite shown in the Part 6 photo, but we're in a different lithology here. In fact, we're back on the bedrock cited in Part 1—the Peshtigo Mangerite. Like the Belongia, it's one of the outlying components of the Mesoproterozoic Wolf River Batholith, and an igneous intrusive rock. But it's distinctly darker—a somber gray or brownish rather than red or pink.

This difference in hue is the result of its different mineralogical makeup. Whereas granites are felsic (rich in light-toned quartz and feldspars), mangerite is a type of monzonite. As such, it's in the intermediate category between felsic and mafic (made up primarily of dark olivine and pyroxenes).

In close-ups to come this color distinction will be more obvious. At this range, however, it's more of a challenge due to the Peshtigo Mangerite's extensively weathered surface and biofilms. An example of the latter is easily discerned in the pale-blue and sulfur-yellow areas, where there are large lichen communities.

You'll find the other photos and descriptions of this series in my Geology of the High Falls Dam Site album.

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

Taken on: May 19, 2002

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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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