Craggy Triptych EarthCache
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The Castle Crags Vista Point is comprised of a suite of three distinct views of geological interest rather than one panoramic one. Geology buffs can start their lesson at the Vista Point, a 1-mile drive from park headquarters, then a 100-yard walk.
If you were standing in this spot 300 million years ago, you'd probably be more than a mile beneath the ocean's surface. The Klamath Mountains were formed when the sea floor slowly crumpled and was thrust upward as two great tectonic plates--the North American and Pacific plates (miles-thick slabs of crust floating on the earth's mantle)--butted and overlapped each other, an ongoing process.
Mt. Shasta – This magnificent mountain is a huge composite volcano that dominates the skyline of North California from Redding to the Oregon border. Shasta is the second tallest mountain in the Cascade Range and the one with the largest volume. According to Norris and Webb, Shasta consists of at least four overlapping cones all of Quaternary age. The main mass of the mountain shows the effect of Pleistocene glaciation, but the summit cone, Hotlum cone, and Shastina do not. Recent studies have shown that the Hotlum cone is the most recently active area on Mt. Shasta. It's believed that Shasta last erupted in 1786, an event that may have been seen by the explorer La Perouse off the Pacific coast.
Grey Rocks - The Grey Rocks form the crest of Flume Creek Ridge. Geologically unlike either the Crags or Mount Shasta. The Grey Rocks consist of a great weathered slab of metamorphic rock principally greenstone and slate, that has been thrust sideways over and on top of serpentine. They were formed from ocean sediments that accumulated 200-300 million years ago.
Castle Crags – The main attraction in the Park, the crags are an “unroofed” intrusive igneous pluton or batholith. The crags began forming about 162 million years ago according to Irwin and Wooden (1999), with magma, rising from deep within the mantle that crystallized into granitic rock just below the surface of the earth. Over time, softer overlying rock eroded away, exposing the hard granite. Finally, glaciers wore the granite into fissured crags during the Pleistocene epoch.
Your geological tasks should you choose to accept them are as follows:
1) Determine the relative geologic age (visit link) of the three geologic features from left to right as you face the interpretative sign and describe your geo-logic in your own words.
2) Describe in your own words what is so unusual about a rock sequence (see Grey Rocks) that contains a thrust fault (visit link) ?
3) What geological process accounts for the shape of the crags toward the right side AND
4) Photographically document your visit by posting at least one image of your favorite of the three vistas.
Email your answers to (visit link) . Logs that contain spoilers may be deleted without notice.
Day use costs $8 per car. Castle Crags State Park is 48 miles north of Redding and about 80 miles south of the Oregon border (6 miles south of Dunsmuir). From Interstate 5, take the Castella exit. The drive beyond park headquarters is narrow and steep. Campers and RVs are not allowed.
Resources:
Most of the information you need is on the Interpretative Sign at the Vista Point and your observations.
CASTLE CRAGS INTERPRETIVE ASSOCIATION, Stratigraphic Sequence of Events and Structural Relationship of Rock Units as Shown in Cross-section, Castle Crags area
IRWIN, WP, AND WOODEN, JL (1999) Plutons and Accretionary Episodes of the Klamath Mountains, California and Oregon, US Geological Society, Open-File Report 99-374 (visit link)
HARDEN, DR, 1998, California Geology, Prentice-Hall, Inc., 479 pp.
IRWIN, WP, 2003, A Bibliography of Klamath Mountains Geology, California and Oregon,
listing Authors from Aalto to Zucca for the Years 1849 to Mid-2003, U.S. Geological Survey
Open-File Report 03-306, (visit link)
LANPHERE MA, IRWIN WP, AND HOTZ PE (1968) Isotopic Age of the Nevadan Orogeny and Older Plutonic and Metamorphic Events in the Klamath Mountains, California. Geological Society of America Bulletin: Vol. 79, No. 8 pp. 1027–1052
NORRIS, RM, AND WEBB, RW, 1990, Geology of California, John Wiley & Sons, Inc., 541 pp.
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