Geologic trip to the Pollywog Agmatite, Highway 20

Ain't they cute? These little buggers are all stretched out from heating and magmatic flow.

Stop by a classic North Cascades locality called the “Pollywog agmatite”, which you’ll find posted in a vignette here. This is a roadside exposure, revealed by highway construction, revealing an intrusive breccia. A dark rock that was intruded by light colored trondhjemite (you’ll learn what that is in the vignette) spalled off as little blocks and was incorporated into the intrusion. Some of those dark rock fragments were partially melted and stretched out into long, curled shapes reminiscent of immature froggies. This is a roadside stop you’d visit on the way to another place.


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