Robert Porath: What’s in them thar hills anyway?
Pass the basalt!
In a recent column in Roots to Roofs, a special section of the News-Register, my wife and I teamed up to write about the stream crossing our property. It’s officially called Soap Creek, “soap” referring to underlying soapstone consisting primarily of talc.
Talc, the softest known mineral, can be scratched with a fingernail. When it combines with other minerals to form soapstone, however, the result is so dense it substitutes for granite in countertops.
Soapstone, a metamorphic rock, forms when another kind of rock came under tremendous pressure. But which one?
This question intrigued us, so I investigated further.
I naturally turned to the Oregon Department of Geology and Mineral Industries, called DOGAMI by rock hounds. That’s not to be confused with rock stars or rock bands, with or without heavy metal!
I also owe geologist William Bryan of Bellingham, Wash., for technical assistance.
DOGAMI “develops maps, reports, and data to help Oregon manage natural resources and prepare for natural hazards.” Earthquake, anyone?
Guest Writer
Robert Porath is a retired Willamette Valley Medical Center pharmacist. He is also a pianist and composer with three albums of original music to his credit. In recent years, he has co-authored the “Couple in the Country” column in the “Roots to Roofs” special section of the News-Register with his wife, Marna. His essays have been published in national periodicals. In addition to playing the piano, he enjoys cutting firewood on his Eola Hills property.
Of particular interest to me was the agency’s geologic map of Yamhill County, and, more specifically, the Eola Hills where we live. It shows what’s under the ground.
I don’t know about you, but seldom is my thinking much deeper than garden soil.
The ridge known as the Eola Hills runs roughly 10 miles, north and south, through Yamhill and Polk counties. Its name originates from an abandoned town at the southern terminus of the range on the Willamette River.
The town was founded in 1851 by a man ti whom the area resembled his boyhood home in Ohio, and called it Cincinnati.
Perhaps the only historical note about Cincinnati, Oregon, is that its first schoolteacher went on to become pioneering women’s rights advocate Abigail Scott Duniway. “Offbeat Oregon” columnist Finn John wrote a fascinating biography of Duniway in the July 6, 2018, News-Register.
Prior to statehood, the Oregon Territorial Legislature changed the town’s name from Cincinnati to Eola, after the Greek god of the winds, Aeolus.
Historians debate the reason for the change. Maybe someone moved to Oregon from a windy mountain in Greece!
But on to the subject at hand.
Although eroded by weather, the Eola Hills today crest about 1,100 feet. That’s nearly 1,000 feet above Amity, our nearest town.
Our house lies at 420 feet, and the hill behind us rises to 728 feet. The stream, which transects our place, identifies the low point, at 380 feet.
The DOGAMI map depicts the Eola Hills as a volcanic uplift formed in the Miocene era, about 10 million years ago. It is composed of basalt, the most widely distributed rock in the Pacific Northwest.
Many individual lava flows were immense, covering large parts of the Pacific Northwest. Some flows traveled more than 400 miles.
We live in the Cascadia Subduction Zone. During gigantic earthquakes, the continental shelf beneath our feet, made largely of basalt, is pushed upward by the incredible pressure of the descending Juan de Fuca plate off our coast. This uplifting created our piddling little hill, as well as the Cascade Range to the east, which pushes up more than 10,000 feet.
We are overdue for another destructive quake — the Big One. The U.S. Geologic Service estimates the strength of a magnitude 9.0 earthquake as possibly 100 times greater than the eruption of Mount St. Helens.
This ongoing tectonic action also produced a similar rise directly to our north — Walnut Hill. A quarry between the two rises exposes the volcanic basalt. Additional local features of this type include Parrett Mountain and the Chehalem Hills, northeast of Newberg, and the Red Hills, above Dundee.
While the McMinnville Hills are also made of basalt, they are four times as old, dating back to the Eocene era. The peaks of the hills used to form an ancient island chain off the coast that was pushed onto the mainland. That represents a good example of the power of our regional tectonic zone.
But back to the soapstone on our property.
Uplifting of volcanic rock creates pressure sufficient for basalt to metamorphose into talc and soapstone. We see evidence where Soap Creek has cut the earth down to the basalt bedrock, creating a series of small waterfalls.
Now, to complete the geologic picture of the Yamhill Valley.
At the end of the last Ice Age, a series of cataclysmic events known as the Missoula Floods occurred. Walls of water, rock, sand and other particles swept periodically across eastern Washington and down the Columbia River Gorge.
The surges resulted from the collapse of an ice dam nearly a half-mile tall in present-day Montana, draining a lake half the volume of Lake Michigan. Imagine a 1,000-foot high torrent racing down the Gorge at speeds of 60 miles per hour. Ultimate surfing!
The water backed up, flooding the Willamette and Yamhill Valleys with a huge, if temporary, lake. Actually, the upper reaches of one or more such lakes extended nearly to Roseburg. The silt ultimately settled to form a thick layer on the Valley floor, contributing to the agricultural richness of our area.
The water level reached about 400 feet elevation, slightly below that of our home, but filling the basin formed by our stream. We would have lived on the edge of an island.
Wow, valuable waterfront property!
Glacial Erratic rock, southwest of McMinnville, was carried by an iceberg that melted here. The river transporting it had a flow 10 times greater than all the rivers on earth.
I said I rarely think deeply into the earth. One exception is our well.
When we moved here, we shared a well with the neighbor on the hill. At 180 feet in depth, it drew water from an old basalt aquifer.
Today, this aquifer is being drawn down from overuse. New wells up the hill must be drilled ever deeper, some to depths exceeding 600 feet. The Oregon Water Resources Department now limits new drilling into the basalt aquifer.
When our new neighbors moved in, they tore out a 40-acre cherry orchard and planted — you guessed it — grapes. Because they wanted total water rights to our shared well, we traded our half for a brand new well next to our house.
Our well driller hit water at just 30 feet, squaring almost precisely with the elevations on the DOGAMI map. Now, we draw our water from deep within this “marine sediments” layer, tapping the remnants of the old lake.
After learning what our geology looks like today, I cannot help but wonder what changes the Big One will bring.
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