Monday, April 4, 2011

Details from the No Thoroughfare Trail in Colorado National Monument Part 2

Last post, I focused on the older Precambrian rocks that make up most of the base of No Thoroughfare Canyon.  For this post I'm switching gears, actually more like throwing it in reverse, to look at some of the most recent activity in the canyon; the flood and rockfall deposits of less than a million years ago. That may still sound pretty old, but remember, we were looking at rocks older than 1.4 billion years last week.

Close to the mouth of No Thoroughfare, you come upon this:

The natural habitat of the elusive hiking backpack.   Look at  that camouflage!
My backpack on the ground is about a foot high so the larger boulders in the upper layers are a good foot in length at least. This is evidence of flood and possibly rock fall deposits (hehe, mass wasting). These poorly sorted deposits run up the canyon quite a ways. The sorting classification is based on variation in grain size of the sediments that make up the rock.  There is a system of measure called the Wentworth Scale  that is used in geology to classify the grain size of sedimentary rocks. This will come in handy in a few minutes. The next picture is a bit further up.  

Small cliffs in the mouth of the canyon.
I've highlighted the portions of the poorly sorted gravels in the next photo. Differences in rock size show differences in the environment in which the rocks were deposited.  The faster and stronger the force of the water, the larger and heavier rocks it has the ability to transport.  Each layer of the larger cobbles and pebbles represent a flood deposit, with the fine grained sands representing calmer water deposition.

Showing off a common tool in Geology.  Outlining rocks.
This picture has a very stark contrast between the fine grained sand deposits and the large boulders and cobbles above.  

Do not stand here during a flood.
 The rock piles on the ground have either fallen from the cliff above, or have been transported by another flooding event.
More exciting yellow outlining.
 Each of the deposits above are really just loose sediment.  They have not undergone the burial and lithification necessary to cement the grains together to become a full fledged rock. Lucky for us another deposit in the area, a bit older, has eroded into the canyon and shows what the rocks from similar deposition environment a few hundred million years ago can look like.  These are what the deposits above can become if they are not eroded away before they can be buried.
What the first deposits aspire to be....if rocks have aspirations I suppose.

A pebble conglomerate, and a foreshortened leg.

The last picture I'll leave you with is one I meant to put up last week, but for some reason or another spaced out.   If you ever needed proof the earth loves you, here it is.

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