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Monday, November 21, 2011
More Photo's From The Toroweap Geology Trip
Alice sent along some more photo's from our Toroweap field trip this morning. Thank you Alice!
In this shot we can clearly see the offset on the Toroweap Fault which runs across the photo behind the shadowed cliff on the left. It is the Esplanade Platform on the upthrown side of the fault, while the bench in the center distance is the same feature faulted lower to the west. About 500 feet of offset has been measured on the fault here. Note the cinder cone on the lip of the downthrown block. Magma likely utilized the fault line as a conduit to the surface. Note also the dip towards the photographer on the downthrown block, which rotated as it was lowered.
Here is an example of weathering on the Esplanade surface. The Esplanade Sandstone began life on the shores of a Permian sea, located not too far west of this locality. Sand was deposited on this coastal plain in low-lying dune fields. Subsequent burial put it into silent preservation for hundreds of millions of years and groundwater left a calcium cement between the quartz grains. Upon exposure, rainwater reacted with and dissolved the calcium cement such that hollows like these were formed. Although Alice took this photo to highlight the beautiful heart shape of the depression, she also had an eye for the process that creates these features.
This is Vulcan's Throne after we descended its top and hiked to another cone north of it. The view is to the south and behind the Throne is a 3,000 drop down to the Colorado River. Note the white playa lake deposits that accumulated at the intersection of the cone (erupted about 71,000 years ago) and the scarp of the Toroweap Fault (with movement mostly before the cone was emplaced - there is a small saddle on the east side of the cone that likely represents post-eruption movement on the fault).