Wednesday, May 25, 2011

Spring Field Trip

This past week I took my Introduction to (Historical) Geology class on its annual field trip to the John Day Fossil Beds. We had a gorgeous day (in betwen days of heavy rain), plus his year I identified a couple of new stops that the students really liked, and I think added a lot to the trip.

The first new stop (for me) was a stop identified in the USGS field trip guide to the Geologic Setting of the John Day Country. Less than 1/2 mile down Fields Creek Road is a geologic marker and a road cut through some volcanic ash beds of the Mascall Formation. Students spent a good 45 minutes going through the ash beds looking for leaf fossils and nut impressions.

The other new stop was an outcropping just north of the Condon Visitors center on Highway 19. The Road and River cut through a large outcropping of Goose Rock Conglomerate. I often have a hard time getting students to recognize conglomerate because many of the hand samples I have in lab the clasts are gravel sized. But the Goose Rock conglomerate has cobble sized clasts that are nicely rounded (pencil for scale).

Not only is it an excellent example of conglomerate but it is a great rock to help tell the history of Oregon. The conglomerate itself is mid Cretaceous which makes it among the older rocks in Oregon. The clasts are largely comprised of greenstones, cherts, gabbros, and granite that are early Cretaceous age. The greenstone and granite really help because they represent the oceanic and terrestrial components of a volcanic island arc that accreted onto the margin of the continent and then weathered and deposited in the floodplains of a Mesozoic river.


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