Thursday, March 27, 2008
Last weekend we were walking past this alcove where you can see some pretty fresh looking Wingate sandstone. The exterior of the alcove has varying degrees of desert varnish while the inside looks virtually untouched. There is a nice large dark coating of varnish river right of the water drop off. When we climbed up to the alcove you could see lots of sandstone blocks with different degrees of varnish lying on the ground. As a rule of thumb I have always heard that a good thick layer of varnish takes about 2,000 years to develop. This is handy when trying to date petroglyphs or in this case rock fall.
I was always taught that the varnish is a result of biological action. Looking through my old notes I found thepaper by Dorn and Oberlander (Science Volume 213, 1981) where they state that desert varnish is formed by colonies of bacteria absorbing trace amounts of manganese and iron from the atmosphere and wind blown dust and metabolically precipitate it out as a black layer of manganese oxide or reddish iron oxide on the rock surfaces. So for all of these years I have been teaching my students about the biologically mediated mechanism of desert varnish formation.
Well it appears that there has been some additional research done since my MA in 1989. There seems to be total agreement (as total as one could expect) with the material not coming from the parent rock and that there must be a mechanism to deliver the material to the rock surface.
Just by walking through the canyons you can see that the presence of water is integral to the process of desert varnish. I like the idea of atmospheric water using desert dust as condensation nuclei instead of bacterial action. Iron and manganese are found in the reservoir of desert dust which acts as condensation nuclei for the water vapor. Upon contact with the sandstone walls, the minerals precipitate out onto the rock. The water then evaporates and the excess dust blows away leaving the patina of varnish. I am concerned though as I have quite a reservoir of desert dust in the back of the jeep and I really don't need and minerals precipitating out on the window
This of course still leaves the question about the presence of bacteria, DNA and organic compounds in the varnish. Instead of being a part of the formation mechanism organisms might be coexisting in the developing varnish as it is being developed.
I don't pretend to be a chemist, geo or otherwise I just enjoy the puzzle of the origins of the patterns on the rock. But really, I usually just enjoy the desert varnish as geology as art!
Sunday, March 16, 2008
Last Saturday was an awesome late winter/early spring day and we took advantage of the weather by hiking into Dominguez Canyon on the Uncompahgre Plateau. This Wilderness Study Area has an incredible display of petroglyphs all along a perennial stream, the appropriately named Dominguez Creek. The rock units are typical of the east side of the Colorado Plateau. The trail was constructed on the interface between the unconformable Precambrian "Black Rocks" as everyone calls them and the Triassic Chinle Formation. Just overhead are my two favorite formations in the canyons: the Wingate Sandstone and the Kayenta Formation. The Kayenta being a great caprock to the more easily eroded Wingate. These two units together create some of the more easily identified cliffs in the Canyonlands area not to mention that the desert varnish on the red rock creates some fantastic sunrises and sets (as an example, I made my family camp in canyonlands in mid November so that I could watch the sun rise hit the Wingate/Kayenta cliffs on the morning of my 40th birthday).
The Wingate tends to be a more massive cliff with some great crack climbing as in the Indian Peaks area. The Kayenta, I tell my students tends to be ledgey because it was deposited in a more wet environment. This also makes it easier to climb...in some places.
So, as we were hiking out of the canyon, we hear the baying of a young sheep on the cliffs above us. A group of four bighorn sheep were climbing the cliffs and it appeared that they were teaching two young lambs the finer aspects of crack climbing. The mom and dad?? (no anthropomorphizing here) were standing above the two kids cheering them on with a deep baying. One kid made it with little effort, but the second one was having troubles. We would watch him/her get a running start but never quite making the top. Each attempt ended with a fall back to the starting point and an earful of young sheep baying. We never did see the second sheep make it to the top. In the image, you can barely make out "dad" silhouetted against the skyline and a very small white dot that is kid #2 trying to climb the cliff
Thursday, March 13, 2008
...or at least the man made ones. Last week the Bureau of Reclamation opened up the jet tubes at Glen Canyon and increase the discharge from near 10,000 cfs to 41,000 cfs in an attempt to mimic natural high water spring runoff. The park service is hoping that they get some beach aggregation by moving sand from the stream bed onto the banks and make some sand bars. All this sounds like a fun experiment and I certainly would not mind being set on beach somewhere in the canyon to do some data collecting (sure beats being in a lab) but what I love is all the public accessible data that is created by these events.
In the summers I teach classes to K-12 teachers where we discuss teaching geology, at all levels, even in the elementary grades. I will put the graph above on the screen and ask them what happened. This country has a lot of great K-12 science teachers, but unfortunately many of them have little experience working with real life data or at least data that didn't come from the book. These public data sites are awesome and can be used to teach something at every level. Needless to say, the high school teachers have a slightly different take home message that the pre-school teachers do but they all leave saying that they can use something in their classes.
After a day spent in a lecture hall, we get ready for the field portion of the course where we canoe through some incredible Utah canyons, trying to be particles traveling with the current towards the ocean and coming face to face with some incredible geology...after all, somebody has to do it.
Thursday, March 6, 2008
Being new to the geoblogosphere and not reading and/or posting every day I find that I have come in late to some of the great death defying posts that others have shared. Last summer we were traversing under a pretty rotten cliff face(volcanic breccia) when we heard a rifle-like-shot come from way above and we witnessed a rock fall that featured a compact car sized rock hitting the road where I am standing in the picture. It was pretty fascinating to watch the whole event, so much in fact that even though we had cameras in our hands no one thought of videoing the rock fall or the resulting dust cloud. The rock hit the road with a thud and then bounced off the other side to the valley bottom.
As I say...Geology Happens