Thursday, June 28, 2012

tafoni #2

Continuing my series, we go a short distance west from Grand Junction, Colorado, to view tafoni in Rabbit Valley.
Remnant features can be tough to explain -- they are products of weathering and erosion, created and defined largely by what has been taken away.  Often we can only speculate as to what happened but here in Rabbit Valley, it appears someone has it figured out.
The origins of tafoni -- natural pockets in rock -- often are mysterious, but not here.
Nested tafoni -- smaller pits inside the large cavity above.
The explanation on the sign seems reasonable, as the rock here is a mix of fragment sizes and types.  It is gravelly sandstone from the Late Jurassic Morrison Formation, deposited by rivers roughly 150 million years ago.  Think about wading a river and walking across sand bars and gravel bars.  Here we have sand and gravel bars that dinosaurs walked across, now preserved in rock.
Gravel bars of the past ... tafoni of the future?
Rabbit Valley, home of the Mygatt-Moore dinosaur quarry, is accessible from Exit 2 on Interstate 70, about 25 miles west of Grand Junction. A 1.5-mile nature trail tours Morrison boulders and outcrops just east of the quarry.
"Trail through Time"
Fossilized dinosaur pelvis in gravelly sandstone.
150-million-year-old sand and gravel deposits, now rock.

Left:  Boulders on the dry slopes of modern-day Rabbit Valley were once beds of sand and gravel in rivers where dinosaurs drank. Below: Rabbit Valley back in the day.
Where dinosaurs drank -- for the full story, see Ty Templeton’s Bun Toons.

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