The mafic dike is traced to this side of the street. Mafic dike (black, vertical feature) through the surrounding felsic igneous rock The erosion of the igneous rocks and deposition of the overlying Lamotte Sandstone creates this 1 billion year gap. On the opposite side of the roadcut, the mafic dike is also seen, and the two can be traced to get an idea of the direction the intrusion spread across.īoth the overall igneous rocks and the mafic dike are truncated by the overlying sedimentary rock layer. The next stop was a combined geologic spectacle: a mafic dike through the felsic igneous rocks, and the gap in rock between the igneous rocks and the overlying sedimentary rocks.Ī mafic dike can be seen sticking almost vertically up through the igneous rocks, up until the overlying sedimentary rocks. Stop 4: Mafic dike and the 1 billion year gap Intro geology students hitting rocks with hammers.I'll keep my distance Important lessons on the correct way to break a rock and not the hammer Francois Mountains, mostly consisting of potassium feldspar (pink crystals) and quartz (gray/white crystals) This results in a coarser-grained igneous rock, as longer cooling times result in larger crystals. The rocks formed here are compositionally similar to the rhyolite from Johnson's Shut-Ins, but formed deep underground, as opposed to explosive volcanic processes. Erosion around the fractures produces large, rounded boulders which supposedly resemble elephants. This goes to show how much overlying rock has been removed since then. This type of mechanical unloading creates both vertical and horizontal fractures in the rock, perfect for quarrying. This fracturing is a result of pressure being released from the granitic rocks, which originally cooled deep underground and under high pressures but which are now exposed at the surface. This park consists of a dome of granite which is heavily fractured. The next stop was to visit Elephant Rocks State Park. The Shut-Ins contains many small falls and riffle pools.Ĭrossing the Shut-Ins, we encountered this cross-bedding in the sedimentary rocks above the rhyolite. For this rock, it is the vertical fractures. The rocks of Johnson's Shut-Ins are heavily fractured. This rock type forms during volcanic eruptions. Rhyolite porphyry make up Johnson's Shut-Ins. The reservoir is also shown.Įast Fork Black River, upstream from the Shut-InsĮast Fork Black River, downstream is the Shut-Ins, where the stream gets bottle-necked The East Fork Black River is shut-in, forming a narrow canyon, due to contact with the more resistant igneous rocks. Topographic map of Johnson Shut-Ins State Park. Due to this resistance, a "shut-ins" occurs, or a section of stream which is channeled through a narrow gorge. The Shut-Ins are a result of the East Fork Black River eroding down through the originally overlying sedimentary layers (which are easily eroded) and eventually hitting the igneous rock below, which is more resistant to erosion. Along the way, we were reminded of the dangers posed by the reservoir. There was only one improvement which bothered me.įrom the visitor center we walked up to the actual Shut-Ins. Although these are not local rock types, they are still fun to look at. Many of the ground tiles around the park are inlaid with interesting rock types. This shows the contrast between the igneous rocks and the flat-lying sedimentary rocks. The purple rocks are the local rhyolite, while the overlying rocks are sedimentary. Rock types used for the Johnson's Shut-Ins State Park Visitor Center. One of the interesting details of the visitor center was the types of rocks used in the construction, which represent the local geology. Part of the lawsuit winnings against the company that owned the reservoir have gone to current and future part improvements. The boulders in the field were carried by the floodwaters. The scar from the flood due the failing of the hilltop reservoir. The evidence of this flood is still seen near the visitor center in the form of a land scar, with missing trees and large boulders strewn about the field. This is a popular visitor site in the area, but was closed between 20 due to the Tom Sauk reservoir breaking and flooding the park. The first stop of the trip was to Johnson's Shut-Ins State Park.
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