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.