Minnesota Geology Monday - Minneopa Falls

Minneopa Falls is a highlight of the Minneopa State Park run by the Minnesota Department of Natural Resources (http://www.dnr.state.mn.us/state_parks/minneopa/index.html).

Minneopa is a Dakota word meaning 'water falling twice' representing the two falls within the state park.  Minneopa State Park is a short drive from Mankato (for a little bit of United States history, Mankato is the site of the largest mass execution in the U.S., but that's for another story).
The falls formed when Minneopa creek flows over the Cambrian-aged Jordan Sandstone.  Because of differential cementation of the sandstone, areas with more cement form the top level of the double waterfall.

The upper falls is small, falling six or seven feet.  Once over these first falls, Minneopa creek flows approximately seventy feet before reaching the lower falls.

At the lower falls, the creek falls 40 feet.  This particular waterfall on the creek is the second highest waterfall in southern Minnesota.  The highest waterfall in the region is only slightly higher and is also found nearby.

Minneopa Falls was formed when Glacial Lake Agassiz discharged catastrophically via Glacial River Warren.  Glacial River Warren carved a large valley (250 feet deep and up to 5 miles wide in places) that created various knickpoints that caused rivers or creeks to adjust their gradients to reach this new baselevel.  Glacial River Warren created this new knickpoint at Minneopa Creek approximately 9,700 years before present and during that time, the falls have migrated upstream almost 0.75 miles.

Beneath the lower falls, if the waterlevel in the creek is low enough, you are able to walk quite near falls themselves and examine large sandstone blocks that have fallen from overhead.

Minnesota Geology Monday - Iona's Beach

Iona's Beach is located 43 miles Northeast of Duluth and just 3 miles north of Gooseberry Falls State Park, along the shore of Lake Superior.  The Minnesota Department of Natural Resources has designated area as a Scientific and Natural Area (http://www.dnr.state.mn.us/snas/sna01000/index.html).  Iona's Beach is near the former site of the Twin Ports Resort and is named after the former owner of the resort, Iona Lind.  Located adjacent to Minnesota State Highway 61, the rock beach is approximately 300 yards long, as seen in this Google Earth image.

The beach is comprised of flattened pink rhyolite pebbles, also known as shingles.  The source area of the rhyolite pebbles is a cliff on the northern edge of the beach.  As Lake Superior's waves erode the rhyolite cliff, individual rhyolite shingles are rounded and tranported along the beach.  A basalt outcrop portrudes into Lake Superior on the beach's southern edge.  This basalt outcrop prevents rhyolite pebbles from moving beyond the area and effectively traps the pebbles in this location.

At the northern edge of the beach, the rhyolite cliff is located.  The rhyolite lava flow is associated with the 1,100 million year old Midcontinent Rift System as are most of the rocks of this area of the state.  The cliff is 20-30 feet above the lake level.

The area closest to the rhyolite cliff is comprised of fairly angular pebbles, since these materials have not been transported very far and as such, the amount of time they have been weathered is limited.

Looking south from the rhyolite cliff, you can see the basaltic point that extends into Lake Superior, effectively trapping the rhyolite pebbles.

Closer to the southern edge of the beach, the rhyolite pebbles are much more rounded and smooth, due to the weathering that has occurred as they have traveled from the source area with the assistance of longshore drift in the lake.

Minnesota Geology Monday - Jeffers Petroglyphs

In southwestern Minnesota, between the communities of Jeffers and Comfrey, is a Minnesota Historical Site called the Jeffers Petroglyphs (http://www.mnhs.org/places/sites/jp/).  The earliest carvings at the Jeffers Petroglyphs were done approximately 7,000 years ago by early Native Americans and some carvings extended until recent times.

From the interpretive center, it is a short trail walk to the petroglyphs site.  The rock outcrop that the glyphs have been carved into is no more than a few football fields in size.

Prominent bedrock in southwestern Minnesota is comprised of Archean gneisses and 1,700 million year old Sioux Quartzite.  The petroglyphs have been carved into the Sioux Quartzite.  Throughout the area, the quartzite ranges in color from pink to purple, at this location is is largely pink.  The quartzite is derived from quartz sand.  The source location of the quartz sand grains is the Penokean Mountains, which were located across central Minnesota and into Wisconsin during Proterozoic times.  As these mountain ranges were weathered and eroded, most minerals would have been changed into clay and easily transported away.  The weathering resistant quartz, were transported south and deposited largely by rivers.

Cross-beds and ripples found in the quartzite indicate the direction of movement of the flowing water as the quartz sand grains were being deposited.  The majority of cross-beds and ripples suggest a southward direction.  There are a few locations known of southward-dipping cross-beds found directly beneath northward-dipping cross-beds.  This suggests that the depositional environment would have been near an ocean shore under the influence of tides.

The carvings would have been done by using another rock type of equal or greater hardness than the Sioux Quartzite.  Carvings include birds, turtles, bison, thunderbirds, weapons and a great number of other objects.  The site most likely represents a religious illustration of important aspects of the lives for these earlier Native Americans.

Also found throughout the site are numerous glacial striations in the quartzite, representing evidence of past glaciation during the Late-Wisconsin glaciation.  The glyphs are carved over the striations, ensuring that they are younger than the glacial period before them.

The Jeffers Petroglyphs site is found in the area of the state that once was large prairies before European settlement.  The site is home native prairie grasses and wildflowers and is also one of the few areas in the state where you can find prickly pear cactus growing.

Accretionary Wedge #42 - Countertop Geology & Decorative Stone

The latest Accretionary Wedge, the monthly gathering of the geoblogosphere, is being hosted by Ian Saginor at his 'volcanoclast.com' blog and is asking for 'any countertop or decorative stone that has been separated by humans from it's source.'  While thinking about this topic, I immediately thought of the dimension stone seen while walking the streets of St. Paul, Minnesota to locations or events like the Xcel Energy Center, the St. Paul Winter Carnival, Park Square Theatre or even the Minnesota State Capitol building.

The Google Earth image below attempts to give a sense of location or place for some of the dimension stones shown seen in this post.  Downtown St. Paul is located alongside the Mississippi River.

The old West Publishing Building is faced with 3,600 million-year-old Morton Gneiss at street level.  Type locality and quarry location is found alongside the Minnesota River in Morton, Minnesota.  This particular gneiss is sold commercially as dimension stone under the name "Rainbow Gneiss."

The James J. Hill library is constructed of pink marble from the state of Tennessee.  During his lifetime (1838-1916), James J. Hill was the chief executive officer of the Great Northern Railway while the railroad completed a transcontinental line to Seattle, Washington.  The library is also on the National List of Historic Places.

A recent addition to the city is the Hamm Plaza,which was created in 1992 in the center of St. Paul's entertainment district.  The walkway of the plaza is constructed of at least four different Minnesota igneous rocks - granite, gabbro and anorthosite.  The intrusive red and gray granites (radiometric ages between 1,780 and 1,730 million years ago), are quarried near St. Cloud, Minnesota.  The gabbro and anorthosite are common intrusive components of the Duluth Complex of Mid-Continent Rift System (1,100 million years) along Minnesota's North Shore.

The Church of Assumption, the oldest existing church in St. Paul (1874) is also found on the National Register of Historic Places.  The church and rectory are constructed of locally quarried limestone bedrock.  Underlying most of St. Paul is the fossiliferous Ordovician Plateville Limestone and upon close inspection of these buildings, one can see numerous invertebrate fossils.

Like most states in the United States, Minnesota has numerous companies quarrying rock for use as dimension stone.  Some quarry Morton Gneiss in the Minnesota River Valley, granitefrom the St. Cloud area or the intrusive igneous rocks of the Mid-Continent Rift System, other companies quarry limestone or sandstone from other areas of the state.