Gal Hill: Hitting the Tuff Jackpot
Ephraim, Utah. Research Day 10 (July 19).
Just a few blocks from where we are staying is a tame elk farm, whose southern border is a very small hill called Gal Hill. As soon as we drove by the hill, we knew that we had hit the jackpot. Most all other exposures of the uppermost large tuff in the Green River are covered by slopewash, but Gal Hill exposes the stratigraphy both below and above the large tuff bed that Bill was mapping in the region with GPS. We decided to spend the greater part of a day at Gal Hill, hoping that it might provide clues to changing lake conditions and facies migrations due to the influx of the tuff.
Scenic overview of Gal Hill, a small hill on edge of Ephraim. In the center of the photo, the larger ledge is the resistant tuff.
Closer view of the tuff at Gal Hill. At this locality, the tuff is much more planar that at other localities, and it appears to maintain a constant thickness throughout. Immediately below the tuff bed is a zone of silicified stromatolites, some of which have been slightly deformed.
Above the tuff bed, the silica content of the Green River Formation seems to increase, with a variety of chert lenses, concretions, and thin chert beds of various colors.
At the top of Gal Hill, the Green River Formation has been bent both downward and upward and fractured, separated by vertical chert lenses.
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