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
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:
Staircase Rapids
Upper Goose Lake
Rattlesnake Slope Erratics
Devils Canyon
Saddle Mountains Overlook (Earthcache)
Wallula Gap Overlook
Potholes Coulee
West Bar Overlook
Lake Sacajawea Flood Bar
