The Early Proterozoic
Last Phase of Penokean Mountains
1,800,000,000 - 1,750,000,000 years ago
(1.8 - 1.75 billion years ago)
Unaltered Granite and Rhyolite
The final stages of the Penokean mountain building episode was the intrusion of chemically identical, unaltered rhyolite and granite. These rocks had not been subject to the high temperatures and pressures of the Penokean mountain building episode and were probably intruded just after the last high temperature and pressure events of this mountain building episode. These rocks were mostly buried by Paleozoic sedimentary rocks, but outcrops can be seen in Columbia, Marquette, Green Lake and Waushara counties.
The unaltered granite and rhyolite of this time is quarried as decorative stone and used in monuments all around Wisconsin and the United States. Everyone knows that General U. S. Grant and his wife are buried in Grant’s Tomb, but did you know that the coffins they are buried in are made from Montello Granite, quarried in Marquette County, Wisconsin? Also note, Wisconsin’s state rock is red granite.