For purposes of logging this cache you must answer the following three questions. You also must take a picture of you or a personal item near the basalt without giving away any answers at the posted coordinates.
At GZ you will be able to get up close to the pillow basalts. You may need to expand and explore the area where they are located to fully answer the questions.
1. How many outcropping groups of pillow basalts did you find here?
2. How many colors in the basalt were you able to locate and why do you think they occur in the basalt?
3. In 1805 could Lewis and Clark have walked to this site? Why or why not?
4. Please take the Mandatory photo of yourself or a personal item near GZ and either send it to me when you message the answers or attach it to your found log.
Please send in your answers and photo at the same time or before logging this earthcache. If you are on vacation and cannot send in your answers at the same time please use the write note option to mark your place.
This earthcache is located on Washington State Property and placed with permission.
Note: An earthcache is the magical combination of geocaching and geological discovery. They vary greatly from traditional geocaches in that there is NO PHYSICAL CONTAINER! Once at the Earth Cache site, take in the scenery, the geological marvels or oddities, and read the cache description. You will likely be asked to answer some questions about the site and take a picture. You can email the cache owner or use the Message Center to send answers to the required questions. Please don’t be intimidated about answering the questions. They are not graded and were meant to enrich your visit, open your eyes and provide you with new knowledge and insights about the location. Most importantly have fun!
This Earthcache is placed for you to understand a little bit about the Washington Coastal Range and the forces of nature that shape our environment.
Coast Range:
As you can see in the cross-section, the Coast Range borders the Cascadia Subduction Zone and consists of three main elements: the Hoh Accretion Assemblage in yellow, Siletzia (called the “Crescent Formation” in Washington) in purple, and the post-accretion sedimentary rock in brown. Siletzia is the oldest. It was thrust over the Hoh Accretion Assemblage, which is still being accreted at the subduction zone. The post-Accretion sedimentary rocks were deposited over the top of Siletzia after it was accreted.
Siletzia formed as an oceanic plateau and so is characterized by voluminous pillow basalts. In Washington, it crops out throughout the Coast Range, from the exposures at Cape Disappointment to the Olympic Mountains. Outside of Washington, Siletzia extends southward to Roseburg, Oregon and northward onto Vancouver Island. It’s gigantic.
The bedrock here is Eocene Cresent Formation. Mostly pillowed and massive basalt that was erupted on the seafloor or oceanic islands. In places there are silty interbeds with marine microfossils.
Pillows indicate a basaltic magma that has erupted underwater. Each Pillow has a glassy rind about a half an inch thick.The glassy rind is lava that cooled instantly when it contacted sea water. The pillow cracked and more lava squirted from the crack and another pillow formed.
Sediment accretion and jetty construction is responsible for much of the land that the current-day state park is located on and that you walked on to get to the pilow basalt outcrops. The campground at Cape Disappointment State Park sits mostly on accreted land. The pillars that you are looking at were once sea stacks, eroded remnants of rocks standing offshore. As the coastline grew outward, they became part of the mainland. A look at a geologic map of the area shows how sediment has built up behind North Jetty, forming new land.
The Columbia River is the main source of sediment in this coastal geologic system. Sediment reaches the coast at the mouth of the Columbia near Ilwaco and is then moved, deposited, picked up again, and ultimately stranded on nearby beaches by longshore currents. These currents, which run parallel to the shoreline, are generated when the wind blows diagonally against the coastline, creating waves. Locally, shifting bay channels, construction of jetties, and coastal geomorphology also affect where sand is deposited and where it is removed.
The mouth of the Columbia River is a nightmare for shipping. The discharge is more than enough to support even large freighters as far upstream as Portland, but the river carries vast amounts of sand and silt, and the shifting bars have caused vast numbers of shipwrecks. In an effort to stabilize the shifting sand bars, jetties were constructed in 1886. One extends for several miles from Cape Disappointment. Although the rock wall helps to keep the river channel clear, it also serves as a barrier to the southward movement of sand along the coast north of the Cape. The sand started backing up immediately forming the wide flat beaches that can be explored today.
For purposes of logging this cache you must answer the following three questions. You also must take a picture of you or a personal item near the basalt without giving away any answers at the posted coordinates.
At GZ you will be able to get up close to the pillow basalts. You may need to expand and explore the area where they are located to fully answer the questions.
1. How many outcropping groups of pillow basalts did you find here?
2. How many colors in the basalt were you able to locate and why do you think they occur in the basalt?
3. In 1805 could Lewis and Clark have walked to this site? Why or why not?
4. Please take a Mandatory photo of yourself or a personal item near GZ and either send it to me when you message the answers or attach it to your found log.
Please send in your answers and photo at the same time or before logging this earthcache. If you are on vacation and cannot send in your answers at the same time please use the write note option to mark your place.
Resources:
https://www.pmel.noaa.gov/eoi/nemo/explorer/concepts/pillows.html
https://nwgeology.wordpress.com/the-fieldtrips/pillow-lava-sites-in-washington/pillows-in-the-crescent-formation-cape-disappointment-state-park/
https://geotripper.blogspot.com/2016/07/cormorant-and-pillow-basalt-https://wa100.dnr.wa.gov/olympic-peninsula/rocks-of-the-olympic-peninsula