Beacon Rock is the core of an ancient volcano. The ice-age floods through the Columbia River Gorge eroded the softer material away, leaving this unique geological structure standing by itself on the banks of the Columbia River. "Beacon Rock" was originally named by Lewis and Clark on their expedition to the Pacific Ocean on October 31, 1805. It was near Beacon Rock that they first measured tidal influences from the ocean on the Columbia River. In 1811, Alexander Ross of the John Jacob Astor expedition called the rock "Inoshoack Castle." The rock was known as "Castle Rock" until, in 1916, the United States Board of Geographic Names restored the name "Beacon Rock." Henry J. Biddle purchased the rock in order to build a trail to the top. The trail was built, and in 1935 his heirs turned the rock over to the state for use as a park. Additional development was done by the Civilian Conservation Corps.
From: Scott, et.al., 1997, Geologic History of Mount Hood Volcano, Oregon -- A Field-Trip Guidebook: USGS Open-File Report 97-263 Beacon Rock is a basaltic andesite volcanic neck or plug (SiO2 about 54 percent), as first suggested by Ira Williams (1916). Chemically it is similar to lava erupted from Chamberlain Hill (about 30 kilometers west at confluence of Sandy and Columbia Rivers). Lava flows of similar composition are exposed in the Beacon Rock picnic grounds (P. E. Hammond, oral cummun., 1994), establishing the base of the Beacon Rock volcano (150 meters elevation) and extend of downcutting since Beacon Rock time -- 130 meters or more. An attempt by Rick Conrey to obtain an age from Beacon Rock failed when no radiogenic gas was obtained, probably owing to relatively young age of the unit and high concentration of atmospheric argon in the sample. Uplift along the axis of the Columbia River gorge began well before Beacon Rock time. Pliocene lava flows cap the south wall of the Columbia River gorge, but the base of the Pliocene sequence, which now lies at about 750 meter elevation across the axis of the gorge, includes pillowed lava and abundant palagonite (Williams, 1916), almost certainly an indication that the lava was encountering the floodplain of the Columbia River when emplaced approximately 3 million years ago (ages in Conrey and others, 1996a, b). The amount of uplift since that time, on the order of 500 meters, can be reckoned by considering that the Portland reach of the river was within or nearly in tidewater influence, owing to the proximity of the marine strandline at the present Oregon coast. The buildup of Pliocene lava forced the Columbia River northward, where it eroded a new channel near where the Columbia River Basalt Group laps out against older volcanic and sedimentary strata (Tolan and Beeson, 1984). Increased understanding of Beacon Rock lava and its age may better constrain the uplift history by virtue of a bracketing youngest age and may provide additional insight into the history of downcutting by the Columbia River.
To log this cache you must post a photo of yourself with Beacon Rock from ANY location. It could be from the top of Beacon Rock or from across the river with Beacon Rock in the back ground. The more interesting the photo the better. Also in addition to the photo, explain in your log what Beacon Rock is.