Minnesota Geology Pictures - Folded Banded Iron Formation

Very near the Soudan Underground State Park administered by the Minnesota Department of Natural Resources is what some people call 'the most photographed outcrop in the state.'  This is a pavement outcrop of folded banded iron formation.  The outcrop consists of metallic hematite, red jasper and white chert.  These originally horizontal layers have been folded multiple times.  In some areas, the jasper and chert have fractures filled with milky quartz.

The picture above gives a larger sense of scale for the site.

Frozen Minnehaha Falls

A week ago, a colleague of mine visited Minnehaha Falls and took the picture above to show me.  The bedrock of the Minnehaha Falls area consists of the resistant Platteville Limestone, the Glenwood Shale and the St. Peter Sandstone.  As water falls over the more resistant limestone, it erodes the weakly cemented St. Peter Sandstone at the base of the 53-foot waterfall.

What's interesting to me in the picture isn't how the water has slowly frozen in the waterfall through the course of a Minnesota winter, it's all of the other frozen water to the right and left of Minnehaha Falls.  In between the limestone and sandstone is the Glenwood Shale.  As water soaks downward through the upper layers of soil it comes to this impermeable layer of shale and flows laterally until eventually reaching the eroded valley carved by Minnehaha Creek.  At this point the water cools to the point of freezing, forming a ring of large icicles.

On September 29, 2012, I took a gigapan of Minnehaha Falls (with a very low amount of water).  For a full size version of the gigapan, please click here.

For a more detailed post on the Minnehaha Falls, please click here.

Minnesota Geology Pictures - 800 Million Year Old Unconformity

In the town of Gilbert, Minnesota is an outcrop of 2,700 million year old greenstone.  Lava initially erupted into an ocean was basaltic, after undergoing a weak metamorphosism involving higher pressures and temperatures.  Plagioclase and pyroxene were changed to green minerals like chlorite and epidote.  Deposited on top of the greenstone is the 1,900 million year old Biwabik Iron Formation partially consisting of red hematic chert.  The difference between the two units are 800 million years of time.

Glaciers originating from the Rainy Provenance during Late Wisconsin glaciation removed most of the Biwabik Iron Formation from this location.  While these glaciers came through the area they also left numerous glacial striations seen in these two pictures as well.

Minnesota Geology Pictures - Gunflint Iron Formation & Magnetic Rock

In areas of Northeastern Minnesota, the bedrock is comprised of the 1,878 million year old Gunflint Iron Formation.  The iron formation consists of black or gray chert alternating with black iron-rich layers.  During the late Wisconsin glaciation, the southwest trending Rainy lobe left a large slab of iron formation laying vertical.  The bedding planes of the surrounding iron formation is nearly horizontal, contrasting with the vertical bedding planes of this slab.  The name given to this 25 foot slab is Magnetic Rock (though many areas of the iron formation are magnetic).

For a more picture heavy look at the hike into Magnetic Rock, click here.  The trail is over a mile in length to the rock.

Minnesota Geology Pictures - Sudbury Impact Breccia

At the beginning of 2012, I started what I thought would be a weekly series called Minnesota Monday Geology, with posts regarding different geological topics around the state.  For nine months the series went by well, until the beginning of the school year where my time was taken up with work associated with school, like planning lessons, grading jail, and taking care of my three children at home.  Although I have numerous posts to finish the series somewhere in my mind, as well as several including geology from neighboring states like South Dakota and Wisconsin (including a recent trip to Van Hise rock) and several with gigapans that I've begun taking, I'm starting another series today.

Nearly 1,850 million years ago, the region that is now Sudbury, Ontario was struck by an asteroid at least six miles in diameter.  A record of this catastrophic moment in time is preserved in the rock record along the Gunflint Trail, near the Canadian border in Minnesota.  The impact breccia at this location has been interpreted as a submarine debris flow 450 miles from the impact site.  At the time of the impact, this area of Minnesota was located near the shoreline of an ancient sea.  Found directly beneath the breccia layer is the 1,878 million year old Gunflint Iron Formation, directly above the breccia layer is the 1,850 million year old mudstones and shales of the iron-free Rove Formation.  The breccia layer is almost 7 meters thick, with the lower levels containing angular pieces of Iron Formation several meters across.  The iron formation found in the upper levels of breccia are much smaller.

A more complete post on the Sudbury Impact Breccia can by found here.

Both of my sons went on a hike to the breccia outcrops last June (2012).  It can be a challenging hike through some dense underbrush for 6 year old legs.

My 4 year old was more interested in the leaf that he 'captured' than what was around him, oh well.

The Lower Carver Rapids on the Minnesota River

Historically, the upper and lower Carver Rapids on the Minnesota River stopped steamboat traffic up the Minnesota River during times of low water.  Having the water level on the river low enough to expose the rapids had been described as a rarity, though the rapids have been exposed for much of the past year due to dry conditions found throughout the Minnesota River basin.  My first trip to the rapids took place last fall when the river was at 5.37 feet, according to the closest river gauge, and can be re-visited here.  Because of the lack of snow last winter and continued dry or drought conditions this summer, the river level has continued to fall to a current level of 3.66 feet.

The picture below shows the lower rapids a year ago from the southern bank of the Minnesota River.  Though the upper rapids were exposed at the time, the lower rapids were seemingly just below the surface of the water.

Ten months later, with the continued drop of the Minnesota River, the lower rapids have been exposed.  The picture below was taken from the northern bank of the Minnesota River.

The rapids are created as water flows over the Cambrian-aged Jordan Sandstone.  The bedrock here is one of the very few locations of exposed bedrock in Carver County, Minnesota.

Below is a 1.15 gigapixel image of the lower Carver Rapids taken from the northern bank of the Minnesota River.
To see the full size gigapan, click here.

Minnehaha Falls - In a dry year

Minnehaha Falls is a 53-foot waterfall found near the confluence of Minnehaha Creek and the Mississippi River.  The falls form as the creek flows over the Platteville Limestone caprock to erode the underlying and weaker St. Peter Sandstone.  Between the limestone and sandstone is a thin layer of the Glenwood Shale.  The Platteville, Glenwood and St. Peter are all Paleozoic in age.  An earlier write-up of the geology of the area can be found here.

The month of September 2012 was the second driest month of September on record for the Minneapolis-St. Paul area.  Officially, just 0.30" of rain fell (the driest month of September was in 1882, when 0.27" of rain fell).  Ironically, September 2011 was the third-driest month of September with 0.36" of rain.

The lack of rain has had an effect on Minnehaha Falls, it is nearly dry now.  The source of Minnehaha Creek is Gray's Bay on Lake Minnetonka and in periods of low rain, the outlet to the creek is closed to preserve the lake level.

Without the flowing water, it is much easier to see the sandstone, shale and limestone layers.

See the full size version of the gigapan here.