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Pilot Rock (Jackson County, Oregon)
Pilot Rock is a prominent volcanic plug located in the western Cascade Range near the east end of the Siskiyou Mountains, just east of the Siskiyou Summit near Ashland, Oregon. Rising thousands of feet above the Shasta and Rogue valleys, it is a landmark distinguishable from over away. At 25.6 million years old, it is one of the oldest formations of the Cascade Range. The rock is protected by the Cascade–Siskiyou National Monument and the Soda Mountain Wilderness. Several trails pass near Pilot Rock, including the Pacific Crest Trail.
==Geology==
Rising above the surrounding hills and thousands of feet above the Shasta and Rogue valleys, Pilot Rock is one of the most prominent features of the Cascade–Siskiyou National Monument.〔〔 It was created when andesitic magma broke through a weak spot in the Earth's crust and solidified beneath the surface, forming a plug. The softer rock around the plug slowly eroded away, leaving Pilot Rock as it is seen today.〔〔〔 Similar features include Devils Tower in Wyoming and Shiprock in New Mexico.〔〔 Argon–argon dating shows that the rock is approximately 25.6 million years old,〔〔 making it one of the oldest volcanic formations in the Cascades, but much younger than the 425 million-year-old Siskiyou Mountains nearby.〔
Pilot Rock has a large amount of columnar jointing, created when the magma cooled.〔〔〔〔 The joints are tilted about 20 degrees to the east due to the gradual uplift of the Siskiyou Mountains to the west.〔 The rock also has an unusually high amount of black hornblende crystals, indicating that there was a significant amount of water in the area when it formed.〔〔〔 Extensive talus deposits have accumulated around the rock's base.〔 The surrounding soil is primarily the sticky, eroded remnants of volcaniclastic breccia.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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