In the very left bottom of the topo map below, you can see a three-way intersection, with one road following Quartzville Creek off the bottom, one following Canal Creek ~north, and one running diagonally up the hill between the other two, and toward the center of the map.After leaving Yellowbottom, head up Quartzville road to that three-way intersection and take the middle gravel road (the other two are paved) up the hill.
The spot near the center marked "Gravel Pit" is the location for the photo. It's an anomalously young cinder cone; I'm told there's a spot where you can see that it overlies glacial drift, which constrains the age to the last 1.5 million years or so. This is a puzzle, because most volcanism outside of the High Cascades in Oregon ended by the Pliocene, about 5 million years ago. I don't know if anyone has done more detailed studies on this or other anomalously western post-Pliocene eruptions, and I don't know enough to responsibly speculate about the magma source. So I won't, other than to point out, there it is.
Oh, and Dana's always cool, but especially in this photo. Cinder cones by their nature are very porous and permeable, just piles of loose volcanic cinders. They also have a large mass, so during the winter, they cool slowly, and during the summer, they warm slowly. I imagine if you were up on this hill in the winter, you'd find areas where snow melts off quickly, perhaps immediately, where warm air rises up out of the cone to the ground's surface. But during the summer, cold air rushes out of the base as a noticeable chilly breeze. In fact, the last time I was here, I didn't even walk up the hill from the road, but I could feel the cooler air, at knee level and below, coming down from vents like the one behind Dana in the photo. When you get close to one, as she is here, it can actually be uncomfortably chilly, especially once you're acclimated to hot summer days. I don't know, but I'd like to think that's why she's laughing here.
This is a photo Dana shot of Aaron Barth and me poking through one of several channels cut through the diorite mass, where dikes of basalt have been preferentially eroded out by floods. During high water, these rocks are entirely submerged, a fact that's difficult to grasp during the dry season when one is more likely to visit. But after the great flood of winter 1996, there were some substantial logs, 2-3 feet in diameter, stranded on the high points of these rocks, which have been since removed by subsequent floods. The erosive power of Quartzville Creek must be formidable during such times, which makes it all the more surprising how little the landscape around the falls has changed in the 30+ years I've been visiting the area.
The upper end of the larger and much deeper (15-20 feet) hole is visible as the dark green area in the upper left center of this photo. It's not clear to me just why there's a resistant ledge out to the sheer drop off of the hole, which is quite evident in the FlashEarth view. I suspect the gap between the two prominent knobs of diorite indicates the presence of another, wider, basalt dike- that trend continues as a narrowing notch behind me and to the right. Unfortunately, the last few times I've been up here, it's either been too early in the summer, and too cold, or I've just been so tired that swimming didn't seem sensible or appealing.
This is where we generally have lunch- it's not frequented by others, it's nice to have the music of the water in the background, and whether you like sun or shade, you can find a spot that suits you. On top of that, on the bare rock, you don't need to worry about dirt or bugs very much. You can also contemplate the sights and geology as you munch away. In this case I was contemplating the dike beyond Hollie's feet. It's easy to get the impression that sills are horizontal tabular (sheet-like) intrusions, and dikes are vertical. More properly though, sills are conformal with the structure, and dikes cut across structure. In the case of this diorite, all the intrusions, whatever their orientation, are cutting across it. So I consider them all to be dikes. Note that, even on the downstream, protected side of that exposure, the dike is eroded back farther than the surrounding diorite. I should also point out that this is summer low water. The lack of substantial vegetation, and the abundance of rounded cobbles and gravel, indicates that frequently during winter and spring floods, pretty much everything in this photo is inundated and scoured.
In a very real sense, this spot is the heart of the Quartzville mining district. The most intense and valuable mineralization is still several miles away, around the historic town site, but the intrusive rock here, diorite, was the source of heat that created the hydrothermal system that mineralized the area in the first place. The rock is dated at about 18 million years, so that was when the alteration took place. I suspect a fault created a weak spot that allowed erosion along the fairly linear drop of the falls, but as we'll see later, basalt dikes cut through this area, and are preferentially eroded by high winter and spring flows. It's possible one such dike allowed the erosion of that step.
There are two holes here that are nice for swimming, or "paddling," as we called the mixture of swimming, wading, and crawling around in shallow water when I was younger. The cascade above falls into the upper hole, and you can push yourself back under that tail, still able to see what's going on out on the rocks pretty well. I'd love to just sit there and wait for someone to show up... then spring out with a yell. Probably get myself shot.
The next stop after the quick pull-off to see the Boulder Creek boulder (which, depending on time constraints, I more often than not didn't do with middle-schoolers) is Yellowbottom Falls. This spot is not really visible from the road, but it's about 0.2-0.3 miles up the road from the Yellowbottom Campground and day use area (The latter has parking and vault toilets, and we'd generally stop there first for a bathroom break. If you choose to swim here- highly recommended for people who don't mind their swim on the chilly side- the day use area is here, and can also serve as a spot to change with some privacy.) There's a fairly wide strip of Douglas firs between the road and the creek upstream from the day use area; they gradually thin as the road and creek converge just upstream from the falls. Where the road and the creek are closest, the firs end and there's a patch of alders along the bank. You'll see a number of pull-outs along the creek side of the road, into the firs, and the last one or two before the alders offer the easiest access to this spot. I'll let the importance and nature of this rock simmer until tomorrow, but if the anticipation is too much, you can go read Dana's spoiler.
Yesterday, I was looking at and comparing these two photos to see if I had taken them with a panorama in mind, but there are some clear changes in perspective indicating I'd moved at least a few feet to get a clearer shot of Boulder Creek. They wouldn't have stitched well, and probably not at all. The boulder here appears to have come down that little creek, which is hard to believe. However, in the spring following the floods of 1996, a lower portion of Boulder Creek Road was blocked by a debris flow on my first trip up there. Later in the summer, I went back, and that smaller debris flow had been cleaned up and the road reopened. However, another, much larger, flow had come down the mountainside into Boulder Creek, and utterly removed the road for something like a quarter to a third of a mile stretch. It was a couple years before that stretch was rebuilt. The boulder above, incidentally, didn't budge. The point is, it's clear this drainage can be affected by debris flows during extreme events; the stream itself has a very high gradient, and the mountainsides above it are even steeper.
And why would we want to go up that road anyway? I haven't thought to get any photos myself, but the *other* pyrite stop is in a (probable) hydrothermal explosion breccia there. Nice BB-sized to pea-sized pyritohedrons can be found easily and picked out there, though they seem less common now than in the past. Larger crystal can occasionally be found (up to a centimeter or two) but they're not common, and as a rule, have less perfect crystal form. A 360 by 360 degree "Photo Sphere," by Gary Miller, can be seen here.