Thursday, January 23, 2014

Geology Concepts - Cross-bedding

Continuing with the geology theme...

Yesterday I wrote about the geologic principle of Original Horizontality.  This is the idea that the sediments that formed sedimentary rocks were originally deposited horizontally, even if the rocks are no longer in a horizontal position.  These horizontal layers are known as beds. These beds are typically found over wide areas and are uniformly thick.

However, sometimes the sediments within those beds are not laid down evenly in a horizontal layers.  The sediments within a bed may be laid down in inclined layers.  This process is known as Cross-bedding.  Cross-bedding happens when sediment is moved by water (or wind) currents.  Cross-bedding is often found in dunes, ripples, and sand/gravel bars.  Cross-bedding in sedimentary rock provides evidence of past currents preserved in stone.

Looking at a photo from Pictured Rocks National Lakeshore, most of the beds found in the exposed sandstone are horizontal, just as they were originally deposited.  There are several beds in which the sediments were not deposited horizontally.  These beds show cross-bedding and are evidence of currents moving the sediments around.

Sandstone beds at Pictured Rocks National Lakeshore - Can you find the cross-bedding?

The next image is the same photograph with some of the cross-bedding indicated by tilted yellow lines.  If you look closely, there are several more areas of cross-bedding that I have not marked.

Sandstone beds at Pictured Rocks National Lakeshore - areas of cross-bedding are indicated by diagonal lines
The next photograph is another image of sandstone deposits at Pictured Rocks National Lakeshore.  The horizontal beds are easily visible, as is one section of cross-bedding.

Horizontal and cross-bedding at Pictured Rocks National Lakeshore

Here is the same photograph with the cross-bedding outlined. 

Sandstone formation at Pictured Rocks National Lakeshore - area of cross-bedding outlined
The above photographs are taken at Pictured Rocks National Lakeshore.  Pictured Rocks is located in Michigan's Upper Peninsula along the southern shore of Lake Superior.  This is about a 4 1/2 hour drive from Mid-Michigan. 

Why didn't I just use photos of a rock formation from Mid-Michigan instead?

There is no surface bedrock to be found in Mid-Michigan.  Most of the bedrock in Michigan is covered by glacial deposits of sand, gravel, and rock.  These deposits range from a few inches in some area to up to 1200 feet.  The State of Michigan estimates that the average depth of glacial deposits in Michigan is 300 feet!

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