MN DNR Minerals Education Workshop 2005
Field Trip guide by Dr. Dean Moosavi and Rick Ruhanen
|Imagine traveling down a lonely gravel road in southwestern Minnesota (Cottonwood County). The landscape is basic glacial till. Very few trees except for near farm places, only a few gently rolling hills. A sign next to the road near a "hill" says Red Rock Dells Park. Nearby a picnic shelter and a few trees give no indication of the wonders nearby. Folloing a trail we find a rather nonspectacular Red Rock Creek, a small tributary to Mounds Creek, in the photo at right. A few steps later we are able to see some layers of Souix Quartzite, the important red to purple colored rock that extends from Soux Falls SD to New Ulm MN. It is the bedrock of southwestern Minnesota. The rock, not a true metamorphic quartzite but a quartz cemented sandstone, extends to more than 1000 feet in thickness. Here this small creek has exposed some of the rock in the glacial farmland. Traveling a little further on the trail we find a rather large gorge with a very large waterfall (more than 20 feet) for this geological area. The creek has eroded away what must have been glacial till filling a previous river or even fault zone. This quartz is very hard rock, the hardest in Minnesota. It is not easily eroded by a small creek and the features of this gorge give no indications of glacier action. The ronded features of some of the walls and very large pothole remnants suggest much more violent water that what has happened since the last glacer retreated about 10,000 years ago.|
|The trees and the gorge provided a source of water, wood and shelter from praire winds for both animals and people for thousands of years. Today it is a source of recreation and an opportunity to observe large cross sections of the Souix Quartzite.
The park was recently "saved" a few years ago by a narrow defeat of a proposal to mine quartzite here. The cross section visible here is valuable in the presentation, history, geology and role of these rocks in relation to the Jeffers Petroglyphs National Monument on the next page.
| Visible in the rock are distinct layers, indications of advancing and retreating off shore environments. In many places cross bedding is evident, the result of water transport and deposition of the sand grains. The rock is about 1.6 billion years old, is composed of well sorted and well rounded grains of quartz. The quartz sand is a remnant of granite erosion, being more resistant to weathering than the feldspar and micas. The rounded nature of the grains indicates that it traveled a long distance to get here (being more rounded as it bounced and bumped into other rocks). The compostion and size of the bedrock indicates that a very large mountain range must have been present to supply all of the sand. The ripple marks visible here and many other places and the large amounds of sediments is evidence indicate a braided stream environment typical of rivers emptying into an ocean or large lake similar to the Mississippi emptying into the Gulf of Mexico. Analysis of the ripple marks reveal a current direction of just east of due south. The red to purple color comes from the presence of iron oxides (rust) coating individual sand grains.
Glacial striations in the rock give testomony to the erosion resistance of this rock. The striations also provide evidence of two glacial advances from two different directions, north and northwest.