The Precambrian Rocks
From here you can see Precambrian rocks exposed along the river below the dam. Although mostly covered by a dark gray coating of carbon, their fresh pinkish gray colors show where they have been scoured off by floodwaters. We are looking at rocks which were intruded, deformed and re-crystallized [metamorphosed] deep within continental crust between 1850 and 1800 mya. Erosion removed several miles of Precambrian rocks before sands were deposited on them during Cambrian time about 500 million years ago.
The oldest rocks here are dark gray amphibolites, which are composed of black amphibole and gray plagioclase feldspar. The amphibolites are "cut" by masses of pinkish gray granite composed of creamy feldspars and gray, glassy quartz. You can see unassimilated pieces of amphibolite "floating" in the granite. The resulting comprise rock is called an "intrusion breccia". You will also see younger granites cutting older ones. Some of the granites, called pegmatites, are very coarse-textured, with big crystals of pinkish feldspar and quartz.
Figure 2 --- Sketch of an intrusion breccia with fragments of deformed granite and amphibolite.
All of the rocks show a distinct layering or foliation which is produced by solid-state flowage of crush debris under extremely high shearing stress and/or recrystallization under compression.
Figure 3 --- Shear-folded compositional layering in contaminated granite. Key highlights axial trace of small fold. Location #2
Figure 4 --- Sheared, contaminated granite; Location #3 Note the gray color caused by contaminated of the granite by amphibolitic minerals. Notice that in some places the foliation has been folded [Figure 4]. This indicates that the foliation was produced during an initial high-temperature-pressure deformation and then later deformed. Multiple deformations are typical of Precambrian rocks.
Figure 5 --- A large fragment or "xenolith" of dark amphibolite in coarse granite contains folded granite stringers similar in composition to the enclosing granite. This relationship suggests defromation of the fragment after intrusion of some of the granite Location #4.
Figure 6 --- Veined Precambrian amphibolite xenolith cut by granite which shows deformation fabric – a thin layering called "foliation". Location: #5
Figure 7 --- Laminated dark granite containing much disseminated amphibolite [dark gray]. Some of this granular fabric was produced by intense shearing along a west-northwest trending fault zone after granite intrusion at 1,800 mya. Location #6
Figure 8 --- Granite dikes and stringers cutting highly deformed and altered amphibolite. Note two ages of granite dikes [granite cutting older granite]. Location #7
Created by: Dr. Paul Myers
Geology Department
University of Wisconsin-Eau Claire
In the Memory of Tim McConville
Donated by the McConville Family
Marker is at the intersection of North Main Street and West Yellow Street, on the right when traveling north on North Main Street.
Courtesy hmdb.org
