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 cap the higher grassland 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 40 minutes west of Grand Coulee and a few miles south of Bridgeport, WA off Hwy 17 along Bridgeport Hill Road. Chief Joseph Dam crosses the Columbia River at Bridgeport. 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. Fall is the best time to visit.

West Foster Creek is one of several arms of Foster Creek, a drainage system that empties north to the Columbia River.

West Foster Creek looking north.

West Foster Creek looking south.

Bridgeport Hill Road is located west of Upper Grand Coulee. Blue areas are digitally flooded to 730m, an elevation that some argue was reached by a flood-engorged Glacial Lake Columbia. There's a complicated history to the flooding and scouring of this landscape.

The white and orange stuff is old - the same age as local basalt flows. Think of it as bedrock.

Late Wisconsin rhythmites are inset into ice-beveled and 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, horizontal beds that have characteristics consistent with two different environments - both slackwater rhythmites and floodplain alluvium. Both the boulder gravel and the thin beds occur together, forming a sloping bench inset into a bulkier foundation of Miocene sediments.

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

Moving laterally along the contact between glacial and non-glacial Miocene sediments we find gray, downward-pinching, sheeted, sand-filled clastic dikes. Deformation along the same contact changes character over short distances.

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

The Columbia River Basalts thin markedly here, at their northern fringe, onlapping older crystalline rocks of the Okanogan Highlands. 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 well exposed around Bridgeport/Chief Joseph Dam (i.e. Colville batholith). Granite forms the mountains to the north and underlies the Columbia River Basalt to the south, at least for a ways. A few steptoes can be found poking up through the basalt to the south and east. I don't think basalt underlies the Ellensburg at Bridgeport Hill Road. The sandstones and shales appear to rest directly on weathered pre-CRB basement. If so, the deposit is not not an interbed, though it would correlate to interbeds farther south where the older lava flows are.