Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Several outcrops of a variety of Precambrian rocks occur in Waupaca. They include:
Fine-grained pink granite outcrops along Granite Street. If the name "Granite Street" seems a bit obvious, bear in mind that a lot of people in Wisconsin refer to Baraboo Quartzite as "granite." |
The panorama below shows the park. Both fine-grained dark intrusive rocks and pink granite occur. A large erratic of Waupaca rapakivi adamellite is visible at the left top of the hill. There are undisputed granitic rocks a few hundred meters to the west on Granite Street, metamorphic rocks a couple of hundred meters to the east along the river, and this appears to be a contaminated border facies. It is very similar to the petrology along the northern margin of the Wolf River Batholith at High Falls Dam
A pinkish, granitic phase | |
Dark, fine-grained rock cut by a small granitic dike | |
A somewhat coarser, dark intrusive phase, very similar to the dark border rocks at High Falls Dam. | |
Above and at left are three general views of the outcrops |
This rock type, a perfect example of a rapakivi granite, is
not exposed here (although it is at a small quarry north of
town). At left
is the erratic, below are close-ups of the coarse (2-3 cm) feldspars,
often rounded by abrasion during magmatic flow. Rapakivi granites, named
for a locality in Finland, have plagioclase cores rimmed with potassium
feldspar, and many of these feldspar crystals show that texture.
At right below, note the differential weathering. It is frustratingly hard to get good samples of this rock. Convenient samples are crumbly, solid samples are too big to move. |
|
The outcrops are located in the river and woods northeast of the municipal garage on Water Street. | |
There is a small park on the river bank. The outcrops begin just north of the parking lot. Views of the outcrops are below. | |
This is a view looking north from the bridge. The outcrops begin about at the distance of the truck. |
The outcrops in the river and on the riverbank are pinkish on weathered surfaces. They generally show a weak subvertical N-S foliation | |
Above and left: general appearance of the outcrops in the woods. They are best seen when the leaves are gone and even then, getting at them involves clawing through brambles. |
Outcrops in the woods away from the river show generally uniform texture with numerous small dikes and veins and are probably intrusive. The contact probably runs roughly parallel to the river and just west of it. | |
This outcrop shows a contact and a very obvious xenolith. | |
Uniform fine-grained rocks with dikes | |
Uniform fine-grained rocks with dikes |
Outcrops in the woods closest to the river have a crudely gneissic appearance.
A view of the park. It's reached most easily by walking along the railroad tracks. | |
A trail leads around the quarry. | |
Much of the rock here is pinkish, fine-grained, and uniform. | |
On weathered surfaces the rock sometimes has a banded or foliated appearance. Thus, it is not intrusive, but more likely metavolcanic. These rocks are similar to the outcrops in the Waupaca River. |
Return to Geology of Wisconsin Index
Return to Geologic Localities Index
Return to Professor Dutch's Home Page
Created 3 May 2002, Last Update 11 Jan 2020