Thursday, December 10, 2015

A line in the sand - Mission Creek (07 DEC 2015)

One final post about my recent trip to Mission Creek Woodland Park.

Mission Creek Park is divided into two distinct areas an upland area with sand soils and a lowland area with muck soils.  The lowland is part of the Chippewa River and Mission Creek floodplains.  The transition between the upland area and the floodplain is a steep bluff that is as much as 20 feet high.

While walking along Mission Creek, I noticed an area of the bluff that had collapsed exposing layers of soil on its face.

Fortunately, I had worn my rubber boots so I was able to wade across the river and examine the sand more closely.

The sloping layers of sand (middle of the picture) are an indication that the sand was being moved by wind or water when it was deposited.

One thing that I noticed right away was the presence of cross-bedding in the sand - it was not laid down in uniform horizontal layers.  Cross-bedding is evidence that when this sand was deposited it was being moved by either wind or water currents.  Cross-bedding is commonly found in dunes, ripples, and sand/gravel bars.  The direction of the bedding can be used to indicate the direction of the current.

The cross-bedding was interesting, but I found something else to be even more interesting.

That line of black specks in the center-left of this photo is a layer of charcoal.  A closer view can be seen below.

I dug back several inches into the bluff to see if this was indeed a layer or whether it was a single narrow line.  A single narrow line of charcoal could be caused by the roots of a tree burning deep below the surface.  This was not a line like that - the layer extended back into the bluff.  I did not attempt to dig very far back.  I really did not want to disturb the overlying layers of soil and cause further collapse. 

What could cause this layer? 

One possible explanation is that before the layers above were deposited, the area was lightly forested.  If the forest burned (creating the charcoal), surrounding soils could become destabilized by the lack of vegetation.  Winds then caused the soil to drift, burying the thin layer of charcoal under many feet of sand.  The cross-bedding in the overlying sand makes this theory seem likely to me.

Does anyone else have a better theory?

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