Pilot Rock is an iconic symbol of the Cascade-Siskiyou National Monument. The Takelma people called it Tan-ts'at-seniptha, "Stone Standing Up." In 1841, an enterprising U.S. Navy lieutenant scouting a route from the Columbia River to San Francisco Bay named it for himself: Emmons' Peak. We know it as Pilot Rock, a welcome landmark for weary migrants on the Applegate Trail in the 1850s and for travelers on I-5 today.
Pilot Rock is part of the Cascade Range, a mountain range notable for its string of volcanic peaks stretching from British Columbia to northern California's Lassen Peak. The Monument proclamation refers to Pilot Rock as "a volcanic plug," describing it as "a remnant of a feeder vent left after a volcano eroded away, leaving an outstanding example of the inside of a volcano." Pilot Rock is composed mostly of volcanic andesite and has sheer, vertical faces with classic columnar jointing created by the cooling of its andesite composition.
Recent research regarding Pilot Rock suggests that 25 million years ago, magma oozed through a weak spot in the earth's crust, but did not reach the surface. As a result, some geologists refer to Pilot Rock as technically a "volcanic plug," but NOT as defined in the Monument's proclamation, which evidently uses "plug" and "neck" interchangeably. However "plugs" and "necks" are defined, what they both have in common is erosion. After the softer rock is eroded, the remaining harder volcanic structure stands up in bold relief to the surrounding landscape as the blockish, irregular, columnar structure we see today.
For a fuller description of Pilot Rock geology, maps, and hiking directions, please visit BLM's CSNM website: http://www.blm.gov/or/resources/recreation/csnm/brochures.php