Wednesday, May 2, 2012

Imbricate Bedding and Frisbee Cobbles

I was walking along a cobble river bar and started noting the subtle alignment of cobbles on the surface. It is subtle and I found it hard to capture through a picture, but what I was observing was imbricate bedding.
Subtle alignment of cobbles

Imbricate bedding (Wikipedia)

Seeing this alignment got me thinking about measuring clasts in metamorphic conglomerate rocks. I spent a fair bit of time measuring and recording x, y, and z axis of cobbles in an attempt to determine strain on a past project. The first place I did this the rock was highly stretched with cobbles taking on an almost New York hot dog shape. But at another site many of the cobbles appeared flattened. And keep in mind finding metamorphic cobbles embedded in a formation where all three axis can be measured is not very easy. In looking at the cobbles I was traversing, I began wondering about the statistical significance of those measurements in particular given that initial roundness ought not to be assumed.

The above cobble in a metamorphic rock would give the impression of flattening strain.

Besides measuring strain, though the original alignment of non round cobbles like the one above can be used to determine flow direction. I have only applied this approach once on a project involving a gravel mine deposit.

Not only cobbles show imbricate bedding. 

1 comment:

Scott Schuldt said...

I noticed this shingling on the Stillaguamish and have been pointing out to my hiking partners. Thanks for giving me the science jargon. It's quite obvious right after the water drops.