Arch Rock, a Dynamic Coastal Landform EarthCache
Arch Rock, a Dynamic Coastal Landform
-
Difficulty:
-
-
Terrain:
-
Size: 
(other)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
This is an earthcache and so there is no physical cache to find. Instead, you will get to discover and learn something interesting about the geology of an area. Please adhere to the earthcache guidelines of Leave No Trace – Outdoor Ethics and be respectful of your surroundings. Please only take photos and only leave footprints.
Arch Rock Overlook is located in the Point Reyes National Seashore at the end of the Bear Valley Trail. This earthcache will give you an opportunity to explore part of a beautiful National Park and educate you about the geomorphology and formation of coastal headlands, sea caves, arches, stacks, and stumps.
Arch Rock Overlook Collapse
On March 21, 2015, the arch portion of the overlook collapsed. Unfortunately, this natural event killed one person and severely injured another. For safety reasons, the trail leading to the overlook has been closed permanently by the National Park Service. Accordingly, this earthcache was disabled for a period of time but now has been updated to reflect these changed conditions.
PicView of sea stack through arch of Arch Rock Overlook (June 2010, J5 Crew)
PicView of sea stack through collapsed arch of Arch Rock Overlook (November 2016, J5 Crew)
Arch Rock Geology and Geomorphology
The Point Reyes National Seashore is geologically separated from the rest of Marin County, located on the west side of the San Andreas Fault, which separates the Pacific Plate from the North American Plate. As a result, the geology of the park is very different from the geology on the east side of the fault.
In taking a close look at the rock around the Arch Rock Overlook, the exposed formations reveal both older rock of the Monterey Formation and younger rock known as Quaternary marine terrace deposits. Both are sedimentary deposits but the Monterey Formation rock is much older and harder.
Geomorphology is the study of landforms and the processes that shape them. The Arch Rock Overlook is a small headland or peninsula that projects from the coastline into the sea with several sea caves and a former arch. The headland was formed by hydraulic erosion of Coast Creek, which drains along the southerly side of the headland into the ocean, separating the headland from the coastal cliffs. The sea caves and arch of the headland were formed primarily by the power of wave erosion. In order to form a sea cave, the host rock must first contain a weak zone or spot. In sedimentary rocks, this may be a bedding-plane parting or a contact between layers of different hardness. The formation process usually begins when the sea attacks small cracks in a headland and opens them through the power of wave erosion. Waves pound the headland with hundreds of pounds of force per wave. Where there are areas of weakness, that portion of the rock erodes away, forming indentations. These indentations reinforce the erosion by focusing more energy from each wave into the confined space creating caves. When the cave wears through the headland, an arch forms. Further erosion causes the arch to collapse, leaving a pillar of hard rock standing away from the coast, known as a sea stack. Eventually, erosion will cause the stack to collapse, leaving a stump. This stump is usually a small rock island, low enough to be submerged by high tide. The graphic below depicts this geomorphological sequence.
PicGraphic from Coastal Geomorphology: An Introduction (2008, Eric Bird)
Logging Requirements
Please note that failure to meet these logging requirements may result in log deletion.
In order to log a find for this earthcache, you must send me a private e-mail that identifies the cache name and waypoint (GC4NRXB) and answers to the following questions:
- Based on the geomorphological sequence graphic depicted above, which numbered picture best represents the current stage of erosion of the Arch Rock Overlook headland?
- Per the geology description above, there are two distinct exposed layers of sedimentary rock in the general area of the Arch Rock Overlook: the Monterey Formation and Quaternary marine terrace deposits. Based on your observations, what type of rock do you think the Arch Rock Overlook headland is comprised of? (Hint: It's an older deposit.) Did you observe both types of sedimentary rock deposits found in the area?
- There are two trail signs on a post at N37° 59.437 W122° 48.630. To ensure no armchair finds, what is the total amount of trail mileage listed on both signs at these coordinates?
Go ahead and log your find when you submit your answers to the above questions. Please do not include your answers in your cache log.
Additional Logging Request
Though not a requirement, it would be appreciated if you take a photograph of you and/or your GPS with the collapsed arch of Arch Rock Overlook in the background and post it with your cache log to verify your visit.
Point Reyes National Seashore Information
| Arch Rock is located in the Point Reyes National Seashore. Point Reyes was established as a National Seashore on September 13, 1962, by President John F. Kennedy. From its thunderous ocean breakers crashing against rocky headlands and expansive sand beaches to its open grasslands, brushy hillsides, and forested ridges, Point Reyes offers visitors over 110 square miles to explore, including 33,300 acres of coastal wilderness area. Estuaries, windswept beaches, coastal scrub grasslands, salt and freshwater marshes, and coniferous forests create a haven of unspoiled and undeveloped coastline. It is home to several cultures over thousands of years, the Seashore preserves a tapestry of stories and interactions of people. Point Reyes awaits your exploration! For a map of the park, click on the logo to the right. |
 |
Bear Valley Visitor Center
The Bear Valley Trail to Arch Rock Overlook starts at the Bear Valley Visitor Center, the primary Visitor Center of the Point Reyes National Seashore, which provides an orientation of the park's roads, trails, and human and natural history. Depending on the season, the Visitor Center is open on weekdays from 10 a.m. to 4:30 p.m. or 5 p.m. and weekends and holidays from 9 a.m. to 4:30 p.m. or 5 p.m. This is a great place to visit before your journey to Arch Rock!
Bear Valley Trail
Bear Valley Trail, a former ranch road, is one of the most popular trails in the Point Reyes National Seashore. It passes through a low gap in the Inverness Ridge and follows a gently sloped 4.2-mile route to the ocean. The trail traverses two watersheds, separated by Divide Meadow, the high point of the hike (±200 foot elevation gain). East of Divide Meadow, the trail parallels Bear Valley Creek. West of Divide Meadow, the trail parallels Coast Creek. The trail is mostly shaded beneath a forest of towering Bishop pine and Douglas fir. Near the ocean, the trail emerges from the forest and splits before the precipitous bluffs of Arch Rock. Veer left (southward) on the Coast Trail to the posted coordinates. The round trip is about 8.4 miles total, so plan accordingly. Bicycles are permitted on the first 3.3 miles of the trail and there are two bike racks where you can lock up your bike before hiking the last 0.9 mile to the posted coordinates.
Sources
Point Reyes National Seashore, the National Park Service Point Reyes National Seashore Website
Geology at Point Reyes National Seashore and Vicinity, California: A Guide to San Andreas Fault Zone and the Point Reyes Peninsula, U.S. Geological Survey publication, pp. 102-130, (Document)
Geology of the Point Reyes National Seashore and Vicinity, U.S. Geological Survey map, compiled by Joseph C. Clark and Earl E. Brabb, (Document)
Additional Hints
(No hints available.)