Wednesday, February 13, 2013

Geology of Manitous

Recently I read a "Roadside Geology" book about RI and CT.   There are two huge masses of granite under RI, Esmond and Scituate granite. This granite is foliated, forming outcrops of layered stone that weathers and breaks off in flat pieces.
 Here is a massive outcrop of  Scituate granite.

Here are the foliations of this rock

Some of this stone was used to build a slanted slab wall that is nearby.
These slabs are about three inches thick, and are relatively uniform.  However, occasionally very thin slabs were produced.  These were not used in wall construction, but were often left standing.
The small slab resting against the tree is less than an inch thick.  The larger manitou is about 2 1/2 inches thick at its base, and less than an inch at its peak.
Sometimes these thin slabs were left at the edges of the outcrop used for quarrying. The one in the center is about an inch thick.  Since granite weighs about 150 pounds per cubic inch, a three-foot by two-foot manitou only one inch thick would weigh about 75 pounds.
Therefore, they could be set up some distance from the outcrop where they originated. Here is the same group I showed on 1/2/2013, seen from the side. Since these are standing near the top of a very steep slope and overlooking the slanted stone row,  it is safe to conclude these were placed here deliberately and permanently.
Although the slabs are almost three feet tall, they are very thin and heavily eroded. The smaller one in the background  is also less than an inch thick.
Some of the slabs seem to be nearly completely split from each other, probably by water freezing between the layers.  The slabs in the  stone rows may have been  slanted to prevent water from freezing between the layers and splitting the stone. The amount of erosion on this granite suggests these manitous have been standing in place for hundreds of years.  How can one estimate the rate of  erosion? By comparison to the   dated stones left to erode in cemeteries.
     Here's a gravestone made of foliated granite in a local farm cemetery. It is also extensively split. This type of granite was not used often in grave markers, probably for this reason.
There is no date, but the nearest stones with dates indicate this one dates to the 1700s.  It is not as deeply split as the manitous.

With more examples for comparison, it may be possible to estimate how long these eroded manitous have been standing in place.
     Many of the standing  slabs and manitous I have photographed over the years are thicker than these, often up to 6 inches.  I will write about them in a later post.  Maybe there once were many more  thin manitous, but they have slowly eroded and broken.

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