Downstream, middle of picture - See them? Tilted Layers!
Launch: Barry's Landing, MT. This launch puts you into the river near the middle of the canyons..
----Rock resists change. Pick one up, hold in your hand, and try to bend it. Try to break it in two. Not easy, eh? "The rock is my foundation," "Solid as a rock" - lots of quotes about the solidity of rock. Yet, we have all seen solid rock twisted, folded, fractured, and faulted. So, are rocks really not so solid?
----The bending, breaking and moving of rock is a subsection of geology known as Structural Geology. As a college student, I was fascinated by rock movements. Continuing my education, I went to the University of Wyoming to study the specialty of structural geology for my master's degree. And here I was, fifty-one years later, looking at structures in the Bighorn Canyon along the border of Montana and Wyoming.
----Rocks that have an inclination to bend, in response to irresistible forces, assume -cline names from geologists. If they bend upward into a fold, resembling the letter A, they are called anticlines. Anti- (opposed) -cline (slopes). And then there are downfolds - Synclines. But here, cruising calmly along through the organized flat-lying stairstep of beds rising in the canyon walls a thousand feet or more overhead, suddenly downstream, you spy tilted beds. Now, they don't appear to be an upfold, or a downfold - simply tilted. So steep a tilt they approach vertical. OK? Now what do you call that structure? Well, pretty easy to remember - a monocline. Mono- (same) -cline (slope).
Flat beds left and right, and a monocline dead ahead
-----My favorite question as a kid (used to drive some teachers nuts) and my favorite question to students when I became the teacher. Answer: We really don't know but here's probably the best guess. When the North American continent was being pieced together by random bits of leftover crust, still drifting around on the underlying molten mantle, some of the disjunct terranes were colliding with the western margin of North America and accreting to the continent. These collisions often resulted in great pressure on the brittle continent - forces were transmitted far inland, and the crust many miles from the impact zone would crack or buckle. The tilted rocks in the monocline in Bighorn Canyon were perhaps deformed by a distant collision between our westward drifting continent and a disjunct terrane. Hmmm - think of that a moment - those rocks tilted by a continental collision from perhaps 70 million years ago when today's Nevada (and bits of surrounding states) 'slammed' into and docked onto our continent.
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