Minnesota Geology Monday - Good Thunder Bay

Along Highway 61 on the north shore of Lake Superior, just a few miles before Grand Marais, is a great observation point that overlooks Good Thunder Bay and has an excellent outcrop of volcanic and sedimentary rocks.  The igneous basalt of the Mid-Continent Rift system overlies a red sandstone and siltstone.

The basalt lava flow is named the Terrace Point and is approximately 160 feet thick.  It has been mapped inland from the lake for a distance of 15 miles.  The sedimentary structure beneath the basalt is what makes the site remarkable.  Along the north shore there are numerous instances of interflow sedimentary structures, but this red sandstone or siltstone is the most easily accessible, being found alongside the major highway of the region.

The 130 foot thick red sedimentary unit represents a large amount of time between different lava flows.  This particular sequence is also one of the thickest interflow sedimentary sequences found on the north shore.  It has been interpreted as being deposited by streams, though some areas indicate deposition at the bottom of a lake.

Some locations at the top of the sedimentary sequence show that the top layers were pushed, pulled or dragged as basaltic lava came out of the rift system.

A great gigapan of the location is found at the following website:  http://www.gigapan.org/gigapans/86878.

Minnesota Geology Monday - Mill City Conglomerate

Within Interstate State Park near Taylor's Falls, Minnesota are several unique geologic formations.  One of these is the informally named, Mill City Conglomerate, a basalt boulder conglomerate of Cambrian age.

About a half mile south of Taylor's Falls, near the 'Welcome to Minnesota' sign is a conglomerate outcrop.  Walking an old railroad bed from the road sign towards Taylor's Falls, you come to a notch in the cliff, this is the site of the conglomerate.

The conglomerate was deposited 504 million years ago along the shoreline of a Cambrian sea, quite possibly near basaltic islands.  Fossils of brachiopods found in the conglomerate provide evidence for the ancient sea.  The majority of the boulders of basalt have rounded edges, indicating a high energy environment.  The basalt boulders are cemented together by a tan or reddish matrix of sand and silt.

Near a hiking trail, not far from the park's campground, one can locate the contact between the conglomerate and the underlying basalt.  This contact represents nearly 600 million years of time from the underlying basalt lava flow to the deposition of the conglomerate.  The Taylor's Falls area has at least ten different basalt lava flows that are the result of the Midcontinent Rift System that formed about 1,100 million years old.