Monday, September 19, 2011

A new campsite for geology field class

Last week I spent an enjoyable 4 days floating down a nearby stretch of the Colorado River looking for new places to take my geology-by-canoe class. There is a campsite almost 1/2 way down this particular stretch that has a name made for a geology field class. "Fault Line".

These images show the Kayeta-Wingate combination being bent downward by the actions of the fault. The canyon just to the left of the fold continues upward right along the fault. It is an easy hike with the exception of minor 15 foot pour over that requires a touch of scrambling.

How easy is it to find this fault line? The BLM post identifying the campsite. Unfortunately the site is found on an outside bend and with the fast water of last week the landing was not pretty. We stayed dry but just barely.
The view across the river from the campsite. Although not as dramatic as the big fold in the first image, I think the fault line is easier to see. The right side (upriver) shows the mid-Jurassic Entrada sandstone. Just downstream from the fault is the early-Jurassic Kayenta formation.
As an added attraction, the cobbles on the gravel bar encompass examples from all directions in the Colorado Rockies. And, if that wasn't enough the channel in the fore ground makes for an awesome float. I love having my students become particles and float down stream.

It truly doesn't get any better

Wednesday, September 7, 2011

Serendipitous geology

I can't believe it has been over a month since I last posted anything...well I can believe it I guess. It is a combination of it being summer and constantly on the move and being a bit lazy when it comes to posting.

Last week I was poking around an un-excavated Pueblo III site in the Montezuma Valley near Mesa Verde NP. I am quite used to looking at cliff dwellings all around the 4-corners area, this was the first Pueblo aged, non-cliff dwelling that I had spent any time in. What struck me first was that the Mancos shale does not have much of the favorite building materials- sand stone blocks. The Mancos shale is an incredibly deep layer of marine sediment left over from the Cretaceous Seaway. It is not known for it's sandstone building blocks. However, as the inland sea retreated somewhat, there were deposited three units that can be used for building blocks: the Point Lookout sandstone was deposited followed by the Menefee formation, a coastal plain region and a local source of coal and lastly the appropriately named Cliff house sandstone. The Cliff house is where most of the famous cliff dwellings are found.

However, the Montezuma Valley is dominated by the Mancos shale.

Lucky for the former inhabitants of this little pueblo the Menefee formation, is not only rich in sandstone but has a habit of slope failure creating sandstone deposits on the valley floor. The search for local building materials just got easier. A little bit of driving around the nearby county roads found a potential sand stone quarry.

To be a more complete geology day and completely overshadow the archeology was the discovery of fossil shells in the sandstone building blocks. My knowledge of paleontology is quite limited and I won't even hazard a guess at the names of the fossils we found.

A section of wall peeks out from beneath some vegetation with Mesa Verde in the background.

Some of the pottery shards found near the site.
A chunk of ancient building materials complete with even more ancient fossil evidence.
...and more
A close up of the surviving Pueblo wall.