A few thoughts… About geology and resistance

By Sandy Compton
Reader Columnist

Forty years ago, I brought a friend home who had grown up in a Texas city and never been to the Northwest. We drove north on I-15 to Butte, then west on I-90 to Missoula, then north on U.S. 93 through Evaro and Arlee to Ravalli, where we turned west on Highway 200. From there, we followed the Jocko River to its confluence with the Flathead at Dixon, and then drove down the Flathead to where it joins the Clark Fork at Paradise. 

My friend watched the terrain change from basin and range to lava patches, sage meadows and agricultural fields in the Snake River Plain. She witnessed the long rise and fall over Monida Pass into Montana and the Beaverhead drainage, where the interstate traverses into a broad valley between rocky and forested ranges — the geological model of the rest of our route until the Clark Fork Valley begins to narrow between Drummond and Missoula. 

She was a watchful type, and appreciative of the scenery we traveled through, but she didn’t say much until we got into the Clark Fork canyon between Plains and Thompson Falls. She looked at the cliffs rising abruptly on both sides of the river and asked, “What did this?”

It was a good question, but I didn’t know the answer. I’ve since learned a little. The cliffs of her curiosity were formed by glaciers and then further processed by huge masses of rushing water draining from glacial Lake Missoula. 

Before the beginning of the end of the last ice age, ice carved the Clark Fork River canyon as well as the Bull River Valley. The results are exemplified by Flatiron Ridge at Thompson Falls, the giant slabs of open rock at the mouth of Thompson River, the cliffs where the ancient Bad Rock trail climbed the glacial cut on the north side of the river; and the peaks and cirques of the Cabinet Mountains Wilderness, the Scotchman Peaks and the Selkirk Mountains.

Evidence of the passage of ice and water is everywhere in the Clark Fork Valley and the Pend Oreille Basin. Lake Pend Oreille itself is a glacial artifact; gouged deeper with each approach of the ice sheet. The Hope and Sunnyside peninsulas are monadnocks, chunks of stone the glaciers rounded off instead of shearing them flat. The lakes of the Cabinets and the Selkirks were scooped out by ice. 

In the Cabinets are great slabs of Precambrian stone with scratches left by rocks carried by glacial passage. At the Clark Fork Delta is Johnson Point, an exposed stone cutting edge where the glaciers were divided by their own relentless progress. The right branch sheared off the face of the Green Monarchs and the left ground out the cliffs between Clark Fork and Montana and blocked the river to create a lake that was still hundreds of feet deep where Missoula is now. This lake filled and drained and filled again many times over the course of two millennia, during which the ice sheet proceeded and receded perhaps 50 times before finally melting away.

Ice is a fluid material. It flows around or over what it can’t break and bulldoze away. This and the resistance to glacial erosion of different sorts of rock is demonstrated at a spot in the high reaches of the Scotchmans. Millions of years ago, a near vertical crack formed in the subsurface sedimentary layers, and magma pushed into the fissure. It cooled slowly into a very fine-grained, black basalt.

Perhaps fewer than 25,000 years ago, a glacier peeled off the overlay and exposed that basalt. The ice levered great chunks out of the sedimentary stone but the basalt was not so cooperative. Its tiny grains must be plucked out one by one, and the ice was forced to flow over it, abrading it, surely, but leaving a wall in one place about eight feet high and very visible in contrast to the layers of gray-white sedimentary stone it resides in. It’s about six feet thick and composed of sand-sized crystals welded together by heat. Downhill of it, the rock has been sheared off, resulting in a long ramp following the sedimentary strata. Behind the wall is a depression from which the sedimentary stone was “quarried” by the glaciers, carried over the resistant basalt and swept away. The resulting basin is quite lovely. 

I’ve noticed lately that several places that had conspicuously displayed Trump banners no longer do so. I find this and other signs of resistance very hopeful. The millions of “little folks” who are appalled by Donnie Boy’s antics are like that basalt wall, combined in such a way that we will remain in spite of efforts to push us aside. Maybe, like I have since my friend’s question four decades ago, we have learned a little. 

In addition to learning about geology, Sandy Compton has also been writing books for 40 years. His latest, Travel Compendium: Sixty-One Days on the Road in the West, is available at Sandpoint Books, bluecreekpress.com and amazon.com.