One thing I appreciate tremendously about my education in the Earth sciences is that it has encouraged me to examine more closely objects that most people would consider mundane. Take, for instance, this building. It’s a shabby stone cabin in the middle of a Western Oregon forest. It has some interesting history… It was built in the 1930s by the Civilian Conservation Corps (part of F.D.R.’s New Deal) and it’s contemporary with Mount Hood’s famous Timberline Lodge. Other than that, this small stone and timber structure does little to draw one’s attention, but it does hide a secret that is well worthy of it.
It’s the stone, of course, that is the most interesting facet of this building. This little cabin is part of the Silver Falls Historic District in Silver Falls State Park. While most of the lodges and cabins in the area were constructed of the abundant local basalt, this diminutive shelter was built from a fossiliferous sandstone found in the northwest corner of the park. And when I say fossiliferous, I mean fossiliferous!
The stone blocks are jam-packed with fossil mollusks. They come from the Abiqua member of the Scotts Mills formation, which dates them in the early Miocene, about 20 to 23 million years old. In those times the area that is now Silver Falls State Park was a beach and the Willamette Valley to the west was part of a wide sea. In the distance some peaks of the coast range jutted out from the ocean surface as part of an ancient archipelago. Basalt headlands broke the surf here in the Willamette Sound and in the sands of these local embayments a diverse ecosystem thrived. What remains today are the shells and imprints of the abundant mollusks that were deposited here en masse.
Here are three samples of the diverse finds within the stone. On the left are some unidentified gastropods. My finger may be pointing to an example of Neverita jamesae. In the center, a likely scaphopod, perhaps Dentalium pseudonyma. And lastly on the right is the imprint of a bivalve, possibly Lucinoma acutilineata.
Sadly, the quarry from which this stone was carved is not accessible to the public, but I can show you, roughly, the area that hosts these deposits. In the map to the left, Silver Falls State Park occupies the area just above the scale bar. The main hiking area is in the little crook where the north and south forks of Silver Creek meet. As you can see, that region is dominated by Columbia River Basalt flows. But to the northwest there is a region of heavy landslide activity (the green) and a long stripe of light yellow. The lightest yellow is the Abiqua member of the Scotts Mills Formation (sedimentary rocks of early Miocene age). This is where the stone quarry lies. The darker yellow is the Marquam member of the Scotts Mills Formation which is from the late Oligocene and a few million years older than the stone we’ve examined.
If you ever have the chance to visit Silver Falls State Park, don’t miss the opportunity to examine the ancient history that’s right in front of your nose.