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Rattlesnake Slope Erratics and Bergmounds

A cache by Bruce Bjornstad Send Message to Owner Message this owner
Hidden : 10/09/2004
2 out of 5
3.5 out of 5

Size: Size: regular (regular)

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Geocache Description:

Cache consists of an ammo box 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, for more information.

Cache Elevation: 805 ft

General Directions: From SR 225 turn off into parking lot for Rattlesnake Slope Wildlife Area between MP7 and MP8, six miles north of Benton City.

Rattlesnake Slope Erratics and Bergmounds

Erratics were brought here by the great Ice-Age floods. An erratic is the name given to rocks that are foreign and don’t belong in an area. In the Pasco Basin are thousands of light-colored granitic boulders, unlike the only natural rock in this region, which is dark-colored basalt. Since we know the ice sheet never extended this far south during the Ice Age, there must be some other explanation for these misplaced boulders.

Geologists now know these erratics floated in on icebergs during cataclysmic Ice-Age floods. Icebergs from the breakup of the ice dam that created Glacial Lake Missoula were carried downstream along with the floodwater. Imbedded into the icebergs were rocks and soil that floated along within the icebergs. These icebergs collected in slackwater areas downstream. After only a few days the floodwaters receded, and many of the icebergs became grounded against the sloping valley walls. Isolated erratics probably represent “dropstones” that melted out from free-floating icebergs. Erratic clusters formed from the grounding of smaller icebergs, and bergmounds - larger piles of erratic debris, developed from larger, grounded icebergs.

Geologists have found numerous erratics, erratic clusters, and bergmounds on Rattlesnake Mountain, because icebergs tended to migrate to, and concentrate in, slackwater areas such as Rattlesnake Mountain. Most erratics found in this area are smaller than a suitcase, but a few are bigger than a Volkswagen Beetle! Granitic (i.e., granite and granodiorite) erratics, the most common, are easy to spot on Rattlesnake Mountain because of their light color. They stand out against the dark, local basalt and sparse, low-growing vegetation. Other exotic rock types that make up erratics on Rattlesnake Mountain include quartzite, diorite, argillite, schist, gneiss, and gabbro.

On the north slope of Rattlesnake Mountain, erratics and erratic clusters have been discovered up to an elevation of almost 1200 feet. The cache lies within a cluster of erratic boulders (below) composed of one rock type called argillite. The home for this rock is hundreds of miles away in northern Idaho or Montana. The largest of the erratics in this cluster is an impressive 12 ft long. Nearby are also a number of low mounds covered with erratic debris; these are bergmounds.

The rocks in this cluster have an especially long and complex history. They were originally laid down in an ancient seabed more than a BILLION years ago! After being buried under thousands of feet of younger sediment they hardened into an especially strong metamorphic, rock composed of silica, called "argillite". Later, the rocks were exhumed during uplift and erosion of the Rocky Mountains. Very recently (in geologic time) the ice-age glacier that blocked Glacial Lake Missoula picked up and transported these boulders southward at glacial speed. These rocks were rafted one more time, very suddenly to this spot, in a floating iceberg, after the failure of the ice dam that held back Lake Missoula. Here the erratics have rested, undisturbed, for at least the last 13,000 years, the age of the last Ice-Age flood.

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
Frenchman Coulee Rib
Devils Canyon
Saddle Mountains Overlook (Earthcache)
Wallula Gap Overlook
Potholes Coulee
West Bar Overlook
Lake Sacajawea Flood Bar

Additional Hints (Decrypt)

[Warning: stay clear of firing range, which is used a lot for target practice on weekends and holidays.]

Decryption Key


(letter above equals below, and vice versa)

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