I have grown up in Oak Grove/Milwaukie and it's my home, so I decided a nice Earthcache would be appropriate to celebrate my geocaching anniversary. I started caching on 7/10/2009 - at a family camping trip at Fort Stevens. My brother introduced me.........and, well, I love it!
Elk Rock Island was part of the original donation land claim of Milwaukie pioneer Lot Whitcomb, and was known as "Lot Whitcomb Island" during the 1860s. It went through six owners before a Scottish grain exporter, Peter Kerr, acquired the property in 1910 from the Rock Island Club (who operated a dance hall on the island, which burned and closed back in 1916). He gave the island to the City of Portland in 1940 with the requirement that it be preserved in its natural state. It consists of 13.24 acres, and is a sanctuary for many wildlife species, including bald eagles and osprey. Elk Rock Island is really a "part-time" island, as it only appears to be an "island" when the river water is high. During low seasons, you can walk right from the Spring Park Natural Area out on the basalt ledge and onto the island.
That weird bedrock you are walking on is really lava flow from very old volcanic activity that happened about 40 million years ago (in comparison, most of the surface rocks around town, including the Columbia River Basalts, are only 10-20 million years old). The lava flows that formed the predominant bedrock are known as "Waverly Heights Basalt" and are about the oldest rocks you can see without driving a good distance out of the Portland Metro area (or digging down really deep). In fact, when building the Sellwood Bridge, engineers found Waverly Heights basalt at depths of up to 150 fee below the bottom of the Willamette River. It is Eocene Basalt. Eocene is a major division of the geological timetable, and is marked by the emergence of the first modern mammals.
During this time the coastline for Oregon was located near Idaho - this place we know as home was under the ocean. The basalt rocks were the ocean floor and were formed due to plate tectonics. The entire Willamette Valley during this time was a marine shelf environment and was accumulating sands, mud and marine sediments. As time progressed, the shoreline moved to the west (mostly due to the uplift of the Coast Range and the formation of the Cascade Range - creating a "trough" between them). A series of volcanic explosions began (for more information, research the Columbia River Flood Basalts), and several more geological events created the whole area into what it is today (see Boring Lava, Missoula Floods). Here is an example, which includes a diagram of the Columbia River Flood Basalts.
Imagine walking on the ocean floor 30-40 million years ago! All you see around you is a field of basalt. Basalt is one of the most common rock types in the world - and the largest occurrences are in the ocean floor. It is fine grained due to the fast cooling of lava at the surface. By definition, basalt is defined as containing by volume less than 20% quartz and less than 10% feldspathoid, and where at least 65% of the feldspar is in the form of plagioclase (a consistent mineral in the earth's crust). The crustal portions of oceanic tetonic plates are composed predominantly of basalt, produced from the upward motion of mantle below ocean ridges. The shape, structure and texture of a basalt is diagnostic of how and where it erupted: whether into the sea, in an explosive cinder eruption, or as creeping lava flows.
For this earthcache, you are not only going to see ocean floor basalt, you are also going to test its hardness - because, well, these rocks rock! You can do this in the field or at home (if you do this at home, you'll need to find a loose piece of basalt at the coordinates - which may not always be available). You'll need a few household items for your experiment: (1) your fingernail, (2) a penny, (3) a butter knife, and (4) a steel file. Hardness testing is an easy test to determine the relative abrasion resistance of different minerals and rocks. Using an index material, like your fingernail (which has a hardness of 2.5), if your fingernail can scratch the surface of the rock, your rock has a hardness of less than 2.5. However, if the rock can scratch the surface of your fingernail, then the rock has a hardness of more than 2.5. By using a variety of common objects for comparison, you can easily approximate the hardness of your rock. Here are the resistance numbers for your items:
Fingernail = 2.5
Penny = 3.5
Butter Knife = 5.5
Steel File = 6.5