West Foster Creek - Bridgeport Hill Road, WA

Steep cutbanks along West Foster Creek expose glacial deposits inset into a thick package of Miocene Ellensburg Fm. Till and haystack boulders left by the Okanogan Lobe caps the higher surfaces. Sediments at the contact between a Pleistocene gravel and a Miocene sandstone are conspicuously deformed in two different ways - vertical clastic dikes and swirly flame structures. The tidy canyon is located a few miles south of Bridgeport, WA (Chief Joseph Dam across the Columbia River) along Bridgeport Hill Road. Jump the guardrail and explore. Decent parking along the sandy roadside. Look for footpaths leading down to the creek and informal boulder crossings. Wear sturdy boots.

West Foster Creek is one of several arms of a drainage system that flows north to the Columbia River at Bridgeport.

West Foster Creek looking north.

West Foster Creek looking south.

Bridgeport Hill Road is located well west of Upper Grand Coulee. Blue areas are digitally flooded to 730m, an elevation that some argue was reached by Glacial Lake Columbia. There's a more complicated history to the scouring of this landscape than the draining of a glacial lake at its highstand.

Late Wisconsin rhythmites are inset into till-capped Miocene Ellensburg Fm.

Soft sediment deformation (flame structures) occurs in places at the contact between Miocene sediments (oxidized sandstone of the Miocene Ellensburg Fm) and a Late Pleistocene boulder gravel. The gravel appears to have been rapidly-deposited by a flood. Atop the gravel lie a number of thin, gray beds that have characteristics from two different environments - both slackwater rhythmites and floodplain alluvium. Both boulder gravel and the thin beds occur together, forming a sloping bench inset into a bulkier Miocene foundation.

This is the contact of interest. Not deformed here, but search around. You'll find something.

Moving laterally along the contact between glacial and Miocene sediments we find gray, sheeted, sand-filled clastic dikes from above are found. So deformation at the same contact changes character within 100m (vertical dikes and soft sed def).

Ice of the Okanogan Lobe left boulders strewn about the landscape. A clastic dike descends into an oxidized sandstone layer (Ellensburg Fm).

The crossbedded fluvial sandstones, deposits of the ancestral Columbia River perhaps laid down near the confluence with paleo-Foster Creek, are composed of reworked gruss, stripped from the surface of deeply-weathered granitic bedrock common in the area around Bridgeport/Chief Joseph Dam (i.e. Colville batholith). Granite emerges to form mountains to the north and underlies the Columbia River Basalt to the south. A few steptoes can be found poking up through the basalt to the south and east. I don't think an older basalt occurs beneath the Ellensburg sediments at Bridgeport Hill Road. Sandstones and shales appear to rest directly on weathered pre-CRB bedrock. If so, the deposit is not not an interbed, though they would correlate to interbeds to the south. Recall, the CRBs thin markedly here, at their northern fringe, onlapping older rocks of the Okanogan Highlands.