Archway Cliffs (Mendo Coast) Earthcache EarthCache
Archway Cliffs (Mendo Coast) Earthcache
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As an earthcache, there is no “box” or “container” to discover. Rather, with this cache, you discover something about the geology of the area. For more info, consult www.earthcache.org.
Logging Requirements:
Send the answers to #1-#4 to me through my geocaching profile. DO NOT post the answers to any logging requirements on this site.
1. List the name “GC25454 Archway Cliffs (Mendo Coast) Earthcache” in the first line of your email. Also, list the number of people in your group.
2. How many arches are visible from this vantage point? If there is more than one arch, then tell me which one has the largest opening.
3. Based on your reading of the description, how are the arches formed?
4. What COLOR and TEXTURE are the rock cliffs?
5. (optional) Pictures that don't spoil the answers for others are always welcome.
I will only respond if you have incomplete logging requirements. Go ahead and log your cache
This earthcache is located on Lighthouse road on your way to the Point Arena Lighthouse. There is a roadside pulloff big enough for 3-4 cars that allows you to view the cliffs that create the point for the lighthouse.
Geology:
As you look at the ocean cliffs ahead of you, you may notice that these bluffs are significantly different from the marine terrace evident near the town of Mendocino (visit link) Point Arena is west of the San Andreas Fault -- the area from Fort Ross to Alder Creek. (cool map here compliments of Team SageFox - (visit link) is all part of the Pacific plate, and thus of a different rock type than is found along points further to the north. Thus, the rocks in front of you are primarily sedimentary stones that have been uplifted after great turbidity flows have concreted them into compressed layers underwater, then uplifted them along the coast.
As Calweb (ceres.ca.gov) states, “Coastal bluffs are actually the seaward edges of marine terraces, shaped by ocean waves and currents, and uplifted from the ocean floor. Characteristic of the California coast from Mendocino County to San Diego, coastal bluffs are less evident along the Northern California coast where the coastal mountains plunge abruptly into the ocean. Rocky headlands are more prevalent along the Northern and Central California coast but may occur anywhere erosion-resistant rocks are found along the shore.
“Coastal bluffs are composed mainly of sedimentary rocks such as sandstones and shales that are particularly prone to erosion. Grains of quartz, feldspar, and mica compressed into layers of sandstone crumble easily; when wet, shales and siltstones disintegrate, and clays and mudstones, soften and liquefy. Lying on top of the sedimentary deposits of many bluffs is alluvial soil, loosely consolidated sand and gravel deposited by ancient rivers and streams.
“Sea caves, sea stacks, and arches, are created by erosion of less resistant components of coastal landforms. Sea caves are formed by wave erosion where fractures occur in the bluff face. Sea stacks and arches, numerous along the wave-battered Mendocino coast, mark the last stand of more resistant rocks. Erosion of the sandstone cliffs at Natural Bridges State Beach in Santa Cruz created a number of arches; today only one remains, and eventually it too will collapse into the surf.”
“The precipitous cliffs, steep-walled bluffs, and rocky headlands that characterize much of California's coastline are evidence of the ongoing geologic processes that shaped the western margin of the North American continent. Unlike the Atlantic and Gulf coasts of North America, whose gently sloping seashores are the result of gradual submergence of the continent's edges, the sheer walls and elevated terraces of the California coast were created by abrupt faulting and uplift. Bluffs and sea cliffs are a testament to the erosive power of waves, winter rainstorms, and wind, while headlands remain where coastal rock has withstood weathering by these elements.” ~ California Coastal Commission's California Coastal Resource Guide
One of the most awesome sights is to see winter storm waves battering the coast. Waves strike the rocks with tremendous force. Water is massive stuff; a cubic yard of water weighs about a ton! As the waves break, water pressure forces sea water into every tiny crack enhancing chemical weathering of the rock as the water evaporates. Much of the wave energy is focused on the headlands which project out into the ocean. The waves are bent (refracted) around the headland so the force of the wave is directed against the sides of the headland as well as at the point. This leads to erosion along the sides leading to the formation of sea arches.
History:
The nearby POINT ARENA LIGHTHOUSE is one of the tallest lighthouse towers in the west. It is rather dramatically perched on a narrow peninsula. The Point Arena Lighthouse Tower is 115 ft. high. Erected in 1870, it was destroyed by the 1906 earthquake, and rebuilt the following year. It was the first lighthouse to be made of steel and concrete. It is the tallest lighthouse on the west coast that tourists can climb all the way to the top.
Congrads to geojammers & geoknits for co-FTF (First to Finish Logging Requirements) on 3/30/10.
Resources:
Alt & Hyndman, Roadside Geology of Northern andCentral California. Mountain Press Publishing. Missoula: Montana. 2000.
The Geology of Salt Point State Park -- (visit link)
Calweb – California Coastal Resource Guide excerpt. (visit link)
Additional Hints
(No hints available.)