MN DNR Minerals Education Workshop 2005
Field Trip guide by Dr. Dean Moosavi and Rick Ruhanen
|While the quarries are not available to the general public the same formations from the dolomite and limstone to the shale and sandstone are visible at Seven Mile Creek Park on the north side of Highway 169 between Mankato and St. Peter. A county park that is free for public use, Seven Mile Creek Park has a small creek that is often almost dry during the summer. The channel has cut into the bedrock exposing the same kinds of layers of rock visible in the quarries. Layers of very soft sandstone between layers of limestone are very easy to scrape away. Scratching away the muddy top layers reveals a crossbedding pattern in the sandstone formed by moving water. Layers in the sandstone are separated by blue-green layers of glauconite clay. The presence of this clay accounts for the names Blue Earth County and Mankato (a Dakota word meaning blue earth). The crossbedding patters in the rock indicates a near shore beach environment with changes in water flow direction.
These rocks were formed about 500 million years ago when Minnesota was south of the equator in a tropical climate. At this point in the earth's history there were very few land plants so any land would be more easily eroded. The valley exposing the rock is less than 10,000 years old, being formed as the last glaciers melted forming the Minnesota River Valley as we know it today.
The creek valley contains many granite, greenstone and gneiss rocks, most too large to be moved by the river today. These rocks were transported here by glaciers or by the meltwater in the much larger River Warren that today is the Minnesota River Valley.