Cache consists of a clear Rubbermaid container with logbook, pencil, pencil sharpener, standard cover sheet, and flyers of the Ice-Age Floods Institute (IAFI). The IAFI is a non-profit organization devoted to informing and educating the public about these unique geologic events that shaped the Pacific Northwest as recently as 13,000 years ago. See www.iceagefloodsinstitute.org, for more information.
Cache Elevation: 1185 ft
General Directions: From I-90 and Silica Rd turn west down Vantage Rd. Go 1.5 miles to parking lot with port-a-potty.
Frenchman Coulee is one of the most awesome features left behind by the great Ice-Age floods. Frenchman Coulee is a dual coulee and cataract system. Like its neighbor to the north, Potholes Coulee, Scabland floods created Frenchman Coulee when they spilled westward over a low divide across Evergreen Ridge, out of the southwest corner of the Quincy Basin. During initial stages of flooding, the difference in water levels between the flood-filled Quincy Basin and the Columbia River immediately west of Evergreen Ridge approached 700 feet over just a few miles. This incredible difference in water levels caused floodwaters to relentlessly eat away the underlying rock layers. Erosion continued for at least as long as it took for the water level in the Columbia Valley to rise to about 1200 feet, or until the floodwater supply was exhausted.
The uppermost basalt layer of Frenchman Coulee lies on the flood-scoured surface of the Roza Member. The cache is located between giant columns of this basalt member. The rounded tops of the huge polygonal columns are fun to hop from one to another. Floods eroded away the white Quincy Diatomite, which once blanketed the Roza in this area. Erosional remnants of diatomite are found along the margins of the coulee to the north and south.
Two dry cataract cliffs (waterfalls during Ice-Age floods) are present in Frenchman Coulee. In some places, the two cataracts merge to form a high, single cataract. Elsewhere, a rock bench separates the two cataracts. These cataracts originated at the mouth of the coulee and receded up the coulee during subsequent floods. Another common feature between Frenchman and Potholes Coulees is a flood-scoured rib of basalt that separates two major, amphitheater-shaped alcoves. The cache is located atop this rib of basalt.
Plunge pools are found below some of the cataracts. Beyond the cataracts, coarse-grained flood deposits blanket the bottom of the alcoves to Babcock Bench. The flood deposits were laid down as giant flood bars with deep troughs (fosses) between some of the flood bars and the steep coulee walls.
Huge, house-sized boulders of basalt (Feature "D" above) are scattered along the bottom of Frenchman Coulee and out onto Babcock Bench. Some of these may have been ripped away from the cataract walls and transported short distances during flooding. Others, however, especially those close to coulee walls, may have simply tumbled off the unstable, steep walls into the coulee since the last Ice-Age flood.
Keep an eye open for an occassional light-colored, boulder (called an erratic) carried in on floating icebergs that were swept along with the floodwaters. When the floods subsided some icebergs became grounded, releasing their contents as they melted. Below is an example.
To experience more incredible features left behind by the Ice-Age floods try finding these other geocaches placed by geologist Bruce Bjornstad:
Upper Goose Lake
Rattlesnake Slope Erratics
Saddle Mountains Overlook (Earthcache)
Wallula Gap Overlook
West Bar Overlook
Lake Sacajawea Flood Bar