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Massacre Rocks
Massacre Rocks is a late Miocene (5-6 million years) basaltic eruptive center along the Snake River. Rising basaltic magma encountered groundwater from the Snake River Plain aquifer, causing explosive "phreatomagmatic" eruptions. Among the rock types present at Massacre Rocks are tuff breccias, which originated as wet mudflows on the flanks of the volcanic crater, lava flows, ash and cinder beds, lava ponded in a lava lake, and basaltic dikes of the core of the volcano. The Massacre Volcanics overlie older rhyolite that extends under the Snake River Plain.

Massacre Rocks got its name because of an Indian attack on the Adams wagon train on the Oregon Trail on August 9 and 10, 1862. It was one of a series of Indian attacks on emigrants that summer along both the California Trail and this part of the Oregon Trail. These attacks were perpetrated mainly by the band of Northwestern Shoshoni led by Chief Pocatello. At Massacre Rocks, ten immigrants were killed either in the original attack or in skirmishes following it.

(left) Culvert under the Oregon Short Line Railroad, Old American Falls townsite, (October, 1987).

(center) Wagon ruts of the Oregon Trail just east of Massacre Rocks State Park, south of Interstate 84. Some accounts place the 1862 massacre just east of here (October, 1987).

(right) Beds of fragmental volcanic debris blown out of the Massacre Volcanic center about 5 million years ago. Photo taken southeast of Interstate 86 at the Massacre Rocks exit (April, 1995)


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