Monday, October 15, 2007

Birch Creek lava surprises

Back on the May trip, Jim, Kathy, and I traveled to Birch Creek to check out the lava flow relations. Everyone heard about the 'nipple' and the 'tank tread' but most didn't see them. We got some nice pictures, and I have posted some of them here that point out a relation that I did not catch until today. It appears that there are two separate Bogus Lavas here, not simply an eroded single lava. Whoops.
The Nipple: This is an obvious protrusion of lava that appears to be an erosional remnant of the Bogus Rim lava. The surrounding lava flow surface is covered with a veneer of exotic fluvial gravel, but the top of the nipple has no gravel cover. It may actually be that a younger Bogus flow is inset against both sides of the nipple. This seems likely in hindsight, particularly based on relations at the tank tread.

The tank tread was our first glimpse of the interesting cooling pattern of lava lobes(?) in which the columnar structure wraps all the way around the flow and looks like a tank tread. There is a spectacular example of this at Birch Creek. What I didn't notice when there is that the flow exposed on the east side of the ridge is actually a lower (inset?) flow. This relation is very clear on the aerial photos and the topo map.

Slightly more detailed version that also suggests an equivocal QT age for all things Bogus.

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