Monday, July 9, 2012

Minnesota Geology Monday - Granites of the St. Cloud Area

St. Cloud has the nickname "The Granite City" for good reason.  There were almost one hundred granite quarries in the area over time, but currently there are four in operation in towns like Cold Spring, Rockville and St. Cloud.  Numerous granite plutons are found in the region, together they are known as the East-Central Batholith.  The individual granite plutons have also been informally named, including the Richmond, Rockville and St. Cloud Red, among others.  Radiometric ages of the granites range from 1,800 to 1,750 million years.

The granite is quarried in the area by several companies for use as either dimension stone or aggregate.  Cold Spring Granite maintains several quarries and a processing plant for decorative uses on buildings, countertops, headstones, etc.  The quarry pictured here are from the Rockville Quarry #2, which is sold by the tradename Rockville White

Located just above the granite in the area, and seen well in many quarries, are rounded boulders of granite.  These are corestones.  The granites in the area are jointed, weathering of the granite affect the corners of the joints first, eventually smoothing the corners and producing rounded boulders.  The weathering event most likely took place during the Cretaceous, the same period which formed the kaolinite clays found in the upper Minnesota River Valley.

The region is also swarmed by many diabase intrusions.  These intrusions are easily seen within the Martin Marietta aggregate quarry.

Below is a picture of me standing in front of a diabase dike intruding on a body of St. Cloud Red granite.

Because quarries are privately owned and a potential safety hazard, exploring them without guidance or permission is not allowed.  Quarry Park and Nature Reserve is a public park that allows easy access to many of the features found within the granite throughout the region.  Like the name suggests, the area was once home to almost 30 granite quarries.

The granite present in the park is known as the St. Cloud Red granite.  Most of the former quarries are now filled with water, one is used as a swimming pond that is 112 feet deep.  There are also large piles of quarried rock found throughout the park.

Walking the many hiking trails within the park, it's easy to examine the relationship between the granite and diabase intrusions.

Upon closer inspection, you are also able to find places where pieces of granite were broken off by the intruding magma.  These inclusions of granite are now frozen within the diabase intrusion.