see paragraphs 2 and 3 here. The McKenzie trough is the clearest expression of the western half of the High Cascades graben. 20 comes down a gentle grade here, out onto the flow, then starts a long, slow climb to Santiam Pass, where it crosses the High Cascade Crest. So this is the spot that I point to when field tripping as the transition from Western Cascades to High Cascades.
It's a bit more complex than that, though. I'm told that tephra from relatively recent eruptions covers the ground pretty deep in the area- so when you reach this spot, you've actually been traveling on recent (High) Cascade material for some miles. You could say, "Oh, but if you dig down a ways, you'd be on Western Cascade bedrock." True, but you could say the same thing out on the flow. The fact is, you could say the same thing all the way across the High Cascades- there's Western Cascade rock underlying the whole pile. I don't know whether any section of the fault (or more likely, fault system) that created this topographic boundary (on the western side) has ever been actually observed, or just inferred. And that would actually be the best boundary marker.
As an aside, the corresponding eastern half-graben is very clearly expressed at Green Ridge, north of Black Butte. Black Butte is a large cinder cone that probably formed where it did as magma was channeled along the same fault(s) that formed Green Ridge. From an older post:
Photo unmodified. October 9, 2012. FlashEarth Location same as yesterday, so today I'll put the cross hairs on the spot where we parked- amusingly, there's another car parked there
Is This Your Hat?
2 years ago