By Dave Tucker October 25, 2010
Getting there: This outcrop is along Washington Highway 20 between mile posts 139 and 140, 5.8 miles east of the Ross Dam trailhead, and 1.2 miles west of the Canyon Creek trailhead. There is a large pullout with room for many cars on the river side (north) of the road 0.9 mile east of MP 139- use caution crossing the highway to park if you are east bound! The road cut is across the highway from the long pullout, and is best seen it is in the shade. The outcrop is north facing, so sun hits it only early or late in the day in the summer. Informal trails run head down to Ruby Creek from the pullout, where there are a couple of nice swimming holes; water is kinda cold, but very refreshing. Gold dredgers sometime use this spot.
This trip visits a classic North Cascades locality. North Cascades pioneering geologist Peter Misch called this outcrop the “pollywog agmatite”. Agmatite refers to a mixture of granitic rock (the white rock) and inclusions. This outcrop shows an intrusive breccia- the pollywogs are inclusions of dark rock that were intruded by an intrusion of a granitic rock called trondjemite; more on that below. (Does anyone out there know what this dark country rock
is?) Bits of the dark host rock broke off or were surrounded when magma filled fractures. The brecciated rocks then became inclusions within the magma and mixed with it. In many cases, the inclusions were deformed by heat to produce the schmeeeeeared out “pollywogs”. Some of the inclusions are swirled and smoothed, others are angular and little deformed. The best pollywogs are toward the east end of the 30 foot high road cut; walk along the grass at the base until you see them. The intrusive event is dated by U-Pb isotopes to 48 Ma, which is pretty young for deep rocks of the North Cascades.
Trondhjemite is a leucocratic (light-colored) intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites are sometimes known as plagiogranites. To learn more about ‘granitoid’ rocks, go to an article at About.com:geology. Tonalite is a slightly larger group of granitic rocks that includes trondjemite. Tonalite is an igneous, felsic plutonic rock, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.
The pollywog agmatite is beloved by petrology professors, and, hopefully, their students.This site is mentioned in virtually every published field guide I know of that discusses geologic processes, especially magmatism, in the North Cascades (e.g. Misch, 1977; Haugerud and others, 1994; Miller and others, 2009; a virtual field trip; and another in a NWGS field guide, which is a very good online guide to the North Cascades highway). The agmatite is part of the Ruby Creek Heterogeneous Plutonic Complex in the North Cascades Crystalline Core, a batch of mixed plutonic rocks found in the vicinity of, you got it, Ruby Creek. The rocks intruded into the Ross Lake Fault Zone, one of the strike-slip faults that were active in the North Cascades as the rocks were being uplifted within the crust.
Misch, P., 1977, Bedrock geology of the North Cascades: in Brown, E. H.; Ellis, R. C., editors, Geological Excursions in the Pacific Northwest: Western Washington University, p. 1-62.
Haugerud, R.A. and others, 1994, Late Cretaceous and early Tertiary orogeny in the North Cascades: in Swanson, D.A. and Haugerud, R.A., editors, Geologic Field Trips in the Pacific Northwest: University of Washington Department of Geological Sciences, 2 v.
Miller, R.B., and a host of others, 2009, Linking deep and shallow crustal processes in an exhumed continental arc, North Cascades, Washington: in O’Connor and others, editors, Volcanoes to Vineyards: Geologic Field Trips through the Dynamic Landscape of the Pacific Northwest: Geological Society of America Field Guide 15, p. 373-406, doi:10.1130/2009.fld015(19). Here is the abstract for the field trip.