The scenic sandstone formation named the Stemilt Formation is on the steep slope above Stemilt Creek road approximately a mile and a half from the Malaga-Alcoa Highway that runs along the west side of the Columbia river. There is no formal sign describing the setting but there is ample space for several cars to park. The formation itself is on private property. Be sure to continue further up the road to see continuing views of this outstanding sandstone formation
The formation is divided into two members, a sandstone and shale member and an overlying conglomerate member, Because basal shale and underlying feldspathic (quartzite) sandstone and shale of the Chumstick Formation do not crop out well on grassy slopes, the basal contact of the Wenatchee Foundation is not exposed. About 15 meters of the conglomerate member crops out at the type section. The contact with the underlying lake beds is sharp and appears slightly discordant, particularly when viewed at a distance.
The shale bed has an exposed thickness of about 145 meters. It consists of thick beds of distinctive tuffaceous shale to siltstone with lesser thicknesses of limonitic and fine quartz sandstone. Conglomerate as it name suggests is a mixture of materials in a matrix of white clay.
The geology of which is the Stemilt Foundation is a part of a greater study by Geologist Randall L. Gresens referred to as the Wenatchee Foundation named for its proximity to the City of Wenatchee, Washington.
Three reference sections in the vicinity are the Dry Gulch, Chopper Hill and Stemilt Canyon. Nearby are Squilchuck Canyon and Blue Grade also a part of the study.
An angular unconformity between the Wenatchee Formation and the underlying Chumstick Formation is well exposed in the Stemilt reference section. The nearly flat lying Wenatchee overlies the Chumstick, that dips 45°. In contrast to the Blue Grade and Chopper Hill reference sections, there is no deeply underlying formation.
At Stemilt Canyon, the basal unit of the Wenatchee Formation is a thick sandstone (approx. 30 meters thick) whereas at the type section and other localities, the sandstone is a typical fluvial quartz sandstone of the type section.
The nearby Dry Gulch Section at one time was an active mining operation that provided high grade silica which was used as a fluxing agent for a Tacoma, Washington smelter.
PLEASE DO NOT MAKE ANY ATTEMPT TO CLIMB ROCKS. THERE IS A GUARD DOG NEARBY TO INSURE YOUR UNWANTED PRESENCE.
Credit: Brent Cunderla, area geologist, USDI Bureau of Land Management, Wenatchee provided the paper “Geology of the Wenatchee and Monitor Quadrangle, Chelan and Douglas Counties, Washington” by Randall L. Gresens from which this information was taken.
To claim this earthcache complete the following BEFORE listing your find:
1. What are the two key members of this formation?
2. What commercial use was Quartz Silica used for?
3. How deep is the exposed shale bed?
4. Submit a photo of you with your GPSr at the Stemilt
5.This cache was adopted over to Mr. Gadget #2. Your answers will be evaluated and IF THEY ARE NOT satisfactory or incorrect, I will be contacting you. Formation.