Imagine Coffee is at 53rd and Philomath Boulevard (Route 20/34) between Corvallis and Philomath. I had never been in there before yesterday, but based on my visit, it's a nice relaxing space, with good coffee, tasty breakfast sandwiches, and a nice selection of pastries. I'm sure they wouldn't mind your business, but if you're in a rush, just zero your odometer as you go through the intersection with 53rd Street.
Why Marys Peak? I know my Oregon Central Coast Range geology pretty well, and there is no place that gives an as complete or well-exposed transect as the drive up Marys Peak. It shows a tale of ocean floor eruptions, building to Hawaiian-style subareal eruptions, with associated erosion and deposition of volcanoclastic sediments on the flanks of that(those) island(s). This was followed by subsidence, accretion to North America, and pretty much at the same time, burial under a voluminous Eocene sediment source in the form of an apparent river drainage coming out of the Idaho Batholith. I've seen speculation that the headwaters of this drainage lay in the lake environment that created the Green River Formation in the UT/WY/CO tri-state area, but I don't know whether that was ever confirmed. While the coarser deltaic facies of the Tyee Formation now lie to the south, in the Roseburg area, in the central coast range, this unit forms well-defined couplets of sandstone-silt/mudstone turbidites. Finally, about 30 million years ago, as the reactivated (after stepping back and reinitiating following the above-mentioned accretion) Cascadia subduction zone got going, a series of Oligocene intrusives invaded the Coast Range. The thousand-foot-thick Marys Peak Sill is an outstanding example of one of these, and its resistance to weathering and erosion make it a capstone protecting the softer underlying rocks, explaining why Marys Peak is the highest in the Coast Range.
Even if you aren't into geology, the drive to the top is replete with wonderful scenery and a parade of ecological change as you drive through ever higher elevations and frequent changes in aspect, leading to large variations in sun and moisture availability. The views from the summit area are to die for. And for geologists, the ability to walk from one side of a parking lot to the other, and see from pretty much directly over the subduction zone, under the ocean horizon to the west, the forearc ridge (where the peak stands), the forearc basin (the Willamette Valley), to the volcanic arc (the Cascades), is utterly mind blowing. You can, from one spot, see a cross section of one of the major features resulting from the plate tectonic activity of our planet.
0.0 Start (Intersection of 53rd and Philomath Blvd-Route 20/34)
0.4 View to north of hills on N side of Corvallis Fault- Siletz River Volcanics
3.7 Turn Left on Route 34, to SW, toward Alsea & Waldport
10 .0 Switchbacks up to Alsea Pass
11.1 (pull off on left; caution crossing opposite lane) Stop 1: Steeply dipping beds of Tyee Formation turbidites.
14.8 (Optional stop- not easy to get more than one or two vehicles off the road here, not recommended for larger groups) Hyalocalstite below (on the right, if you're facing upslope), crudely columnar jointed basalt sill above (left). In between, a few inches of Kings Valley Siltstone, the result of erosion and deposition of Siletz River Volcanics that erupted subareally. This is the lowest (stratigraphically) spot in which I've found it.
See further discussion here. Hand sledge for scale on lower left of outcrop.
See this for a detailed discussion of the unit. Another feature here are some ashy layers. It was suggested in the early 80s that these might be derived from the Challis Volcanics (PDF), and with the clearer picture of the Coast Range block migration that emerged in the mid to late 80s, that seems to make even better sense.
Click here for a selection of sizes at Flicker; right click on that image for a list of size choices. Alternatively, right click on the photo above and open image in a new window.) To the left, you can see the horizontally bedded Tyee Formation, with its well-defined sand-mud couplets. On the right, the poorly bedded and more chaotic Kings Valley Siltstone stands out mostly because it's so much darker- almost black- than the Tyee. Again, we can infer a fault between these two exposures. Another point of interest is that you can see the base of the Tyee above the Kings Valley on the right side (though it doesn't really show up in the above photo)- the only place I think I've ever seen the base of that unit. On the left-most side of the photo, you can see a temporary Jersey Barrier, and the edge of the road has collapsed. This has been repaired, and the barrier replaced with a hefty guardrail, but this failure was very likely due to the fault, which shattered the rock, and allowed easier access for water and air to weather the zone out more quickly and easily than elsewhere.
See Dana's post for more information being explained by me in video format.
22.3 Summit Parking area, Stop 5: views of subduction zone, forearc ridge, forearc basin, and volcanic arc. I can't say enough about this spot. There must be other places in the world where you can basically see from the location of the subduction zone to the associated volcanic arc, but this is the one I know. And it blows my mind every time I get up there on a clear day. Again, here's what Dana had to say about it. Below is a view to the west from near the summit parking area; the dim blue line is the horizon above the Pacific Ocean, and a bit of trigonometry suggests that the horizon is about 75 miles distant- very close to the location of the decollement fault where the Juan de Fuca plate begins its dive under North America!
keep an eye out for cobbles of Marys Peak Sill weathered out onto the surface.