Saturday, October 15, 2011

Volcanic Ramblings Part 4: The East Side Apron

The Oregon Cascades are conventionally divided into two N-S sections: the western Cascades, which start as low foothills on the eastern margin of the Willamette Valley, then rise to quite respectable heights before sloping down to the High Cascades. I know that sounds oxymoronic, but it's an accurate description. On route 20 in particular, one crosses two very well-defined passes. You may recall this diagram from my post on Salt Creek Falls:
The Western Cascades is older, ranging from roughly 35 Ma to 5 Ma. The combination of our wet PNW climate, the fragmentary nature of much of the rock, recent glaciation, with the lack of ongoing volcanism means that the Western Cascades have been deeply incised. Indeed, some of the canyons through the area are awesome, and the Western Cascades overall are far more precipitously rugged than the High Cascades- with the exception of some of the big stratovolcanic peaks.

The High Cascades, as implied above, are younger, from about 5 Ma to present. In a few areas, recent glaciation has cut deep canyons well into the High Cascades platform, but overall, the province is a surprisingly gentle swell, punctuated with a relative few peaks that rise another 5000 feet or so above the ~5000-foot pass levels.

The moniker I applied in the title, "the East Side Apron," is entirely my invention, and is not used in broader literature, as far as I know. However it does a good job of expressing my perception of descending eastward out of the mountains toward Central Oregon: a very smooth and steady decline. There are exceptions, of course, but by and large, the east flank is exceptionally gentle. It gets comparatively little precipitation, so erosion is subdued, glaciation was much more moderate than on the west side, and the prevailing winds are from the west, so tephra-rich eruptions regularly cover the landscape with inches to feet of ash and debris, smoothing it out.

One of the more recent large eruptions in Oregon, that of Mount Mazama, is easily recognized in many road cuts in the central part of the state; when coming down from Willamette Pass on route 58, I always point it out to my traveling companions, and if interest warrants, make a stop. The general area where it becomes too obvious to miss is near where the Crescent cut-off takes off to the east near Odell Butte.
Mazama ash and tephra has a distinct creamy-yellow color; its coarseness varies, becoming finer with distance from Crater Lake. Where roads have been cut into and through it, you can see that it has a consistent thickness in a given location, but that unlike a bed of sedimentary rock, it is not horizontal. It follows the shape of the landscape.
In fact, after bringing this odd layer to Dana's attention, it only took a couple of prompts, "Look at the way it's following the landscape," before she let fly with a squee and wildly started scanning for the first safe pull-out she could find.
(lens cap is ~52 mm) As she notes in her description of this stop, the dark cinders are from winter time road gravel, but there's no problem finding as many pieces of pumice as you want to carry away.
I ended up with maybe 7 or 8 of the little buggers; I wanted to stop later to get a couple of head-sized chunks off the roadside near Chemult, but that ended up not happening. Another time.
The best part of this stop? Dana's non-stop squeeing and exclamations.


Dana Hunter said...

You'd think, after all the amazing stuff we saw, that this stop would fade in glory a bit, but no. First time I'd ever got my hands on Mt. Mazama pumice. That leaves an impression on a person.

Thanks for that! And for this!

Lockwood said...

You're most welcome, ma'am!