Field Trip Stop 1: A Gravel Bar from Lake Manly
There are excellent photos of geologic features in and near Death Valley, and very good descriptions of these features, in the VIRTUAL GEOLOGY FIELD GUIDE, DEATH VALLEY REGION, CALIFORNIA & NEVADA (by Steven Spears of Palomar College, California, click to obtain it as a PDF file).
Its stop 'CD-3' is about this gravel bar, with a better photo than you will see below illustrating the size of it. Now comes the question: what makes a gravel bar? They occur when rapidly moving water (loaded with sand and gravel) slows down by spreading out of a narrower channel (as onto an alluvial fan) or by coming in to a lake, so that its debris is dropped as the water loses momentum. The wave action of the lake can also rearrange gravels, and maybe a combination of inflow deposition and wave action caused the laminar structure of this sandbar.
According to the guide by Spears, Lake Manly was filled to this level about 150,000 years ago, and to a slightly lower level about 110,000 years ago, judging by a lower parallel bar (see pictures below), but was nowhere near this high during the Pleistocene, the last ice age, which ended 10,000 years ago. The Amargosa River only flowed into Death Valley toward the end of the last ice age, Spears says, giving a reference to a paper on the topic by a geologist.
I have a hard time believing that latter statement, since the Amargosa flowed quite impressively into Death Valley in early 2005, after only a few months of triple-normal rainfall. I gave a talk at this Devils Hole Worshop on that very topic: answering the semi-serious question "how close did we come to seeing some ice-age phenomena in the winter of 2004/2005?"
The answer was that we did not come very close at all, but we did see some phenomena seen during ice ages: flow in the Amargosa all the way to Badwater, for example! If you want to look at that presentation in PDF format, click here.
(Since I gave this presentation as part of my work duties, it is a presentation with the Department of Energy's logo on every page. Asked what this has to do with Yucca Mountain studies, the answer was easy: we need to know our geologic and hydrologic setting and the potential effects of climate changes, since 80% of the time into the past the region has been cooler and wetter, and presumable this will be the case again as we move out of the current interglacial hot and dry period and as we overcome the temporary effects of global warming (hotter and maybe wetter, if summer moisture is increased). But the presentation is scientifically lighthearted, meant to be illustrative and not quantitative, and to inspire awe and reverence for this magnificently rich region. It is rich in astonishingly varied geological features. It is rich in springs and seeps and their associated flora and fauna. Occasionally it is rich in surface water accompanied by a riot of flowers. The place is a testament to the resiliency and ingenuity of life on this planet. That is what the presentation was all about.)
So, let's take a look or two at this gravel bar (remember to look at stop CD-3 in Spears' field guide also for more information).
This gravel bar is on the road that branches off California highway 190 to go east to Beatty.
Here is proof I was part of a tour, looking up that road and past the people casting shadows:
Looking toward the east gives the mountain-source for the gravels:
Looking south shows the bottom of Death Valley in the distance, but it also shows a parallel lower gravel bar. Perhaps this is the one from ~110,000 years ago?
Finally, there is nothing quite like a road-cut to show structure inside the gravel bar. There are interbedded gravel and silt/clay lenses. Note the ubiquitous "desert pavement" all around. Spears discusses the two competing theories for thie occurrence: wind-removal of fines leaving rock, or shrink/swell in clays with adhesion basically moving rock up over time. Even if there is little clay at one point in time, clay dusts settling out between rocks over time will result in a thin clay layer that will grow over time (a desert Av horizon) and separate surface rock from lower rock. The latter conceptual model, and its implications for the eventual healing of disturbed pavements, has strong support from desert- pavement healing experiments in Nevada and California. For a reference, click here for an abstract of a more scientific discussion, or here for a plain-language discussion.
Go to SECOND STOP: Keane Wonder Mine/Springs (in 3 parts)
Go to Part 2 of this SECOND STOP (Archean life forms!)
Go to Part 3 of this SECOND STOP (cactus flowers and a tiny waterfall)
Go to THIRD STOP: Monarch Canyon and Springs (in 3 parts)
Go to FOURTH STOP: Some interesting volcanic rocks near Scotty's Castle
Go back to 2006 Yearbook page (Item 13 has additional 2006 Death Valley photo pages)
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