Yellow Aster Butte is arguably the most geologically interesting hike in the Mount Baker area (and that is saying something!). In the course of less than four miles, you walk on remnants of at least three different tectonic plates, or terranes. Some may have broken off from other continents, others might be oceanic island arcs (like Hawaii), but wherever they came from, continental drift has piled them up on the western edge of North America long ago. Now they are all scrunched together, eroded over time, and form the the western portion of the North Cascades.
It is possible that similar formations underlie the Cascades further south, but there the remains of the huge chain of volcanos that built the Cascade Range are what you see. Only up here in the north have the Cascade lavas eroded away almost entirely (the Black Buttes and Table Mountain are exceptions) to expose these “basement rocks”.
I have given a lot of attention over the last ten years to trying to find the actual transition lines between these plates. The heavy vegetation and snow cover, and the extreme complexity of the formations themselves, generally make it pretty difficult for an amateur to find those faults, but this trail has a couple that just shout at you. Well, they whisper really loudly, at least.
The hike starts in the Swamp Creek valley on rocks of the Easton terrane, crosses the Chilliwack River terrane as you go up through the woods and ends on the Bell Pass Melange and the Yellow Aster Complex. As usual, I ran on and on about the geology, but I decided to pluck that stuff out and put it in a little appendix so I could get right to the pictures.
Serpentine hills (and 2 hikers)
The first fault (Bpm - Yac)
As you walk along on the level part of the Yellow Aster trail after splitting off from the Tomyhoi Lake trail, the rocks gradually change from the Chilliwack to the Yellow Aster Complex. The change is gradual because you are walking very close to the fault-line; the Chilliwack is below and the Yellow Aster Complex above the trail, so the rocks you see are pretty jumbled up. At the end of this mostly level stretch of trail, about 2.5 mile into the hike, you come to a series of strange, bare, brown hills followed by a sharp drop into a valley, usually filled with snow. This is the first of two striking fault zones. The brown hills are solid serpentine, which usually shows up near faults. It is also usually green and shiny, almost like jade, but here it is darker, almost black, and is heavily weathered – iron produces the rusty brown color.
It is very common to find stream valleys following fault lines, since the movement along a fault tends to grind up the rocks, making openings for water and roots to get in and do their erosional magic.
Looking up toward the butte, you can also see some brownish serpentine on the face of the notch, just above the center of the photo on the left. This fault separates the Yellow Aster Complex, on the right, from the Bell Pass Melange, but the difference is not at all obvious from a distance.
Approaching the fault
But the really cool, textbook example of a fault exposure comes almost another mile up the trail. The first picture shows Con, Cindy and DJan approaching the fault, which runs diagonally across the scene just above the middle. Notice that on the lower right we are hiking along an even slope through a lot of wildflowers and there are few exposed rock outcrops – and then suddenly it is all bare, craggy rock with only sparse vegetation. The snow puddle at the upper right is lying right on top of the fault, and draining along it right through the big shiny gray patch (which may be more serpentine) near the center of the picture.
On the fault (Yac - Bpm)
Wherever I’ve found the Bell Pass Melange, I’ve noticed that the terrain is comparatively smooth, with few outcrops, and the samples I’ve picked up are always covered with moss and lichens and such. Perhaps there are some really tasty minerals in there, or maybe it’s because of all the nooks and crevices between the various components of the melange, but this stuff seems to weather and erode away easily. On the other hand, the Yac is very old and much harder and more compact.
Looking back down the fault
When we got up on top of that snow puddle the contrast was even more striking – we were picking our way through sharp, bare rocks and a few feet away was a gentle mountain meadow leading up to the top of the butte. It is not unusual for the nature of the underlying rock to affect the vegetation that grows on it, but this is a particularly good example. In the last picture you can see that the snow near the center is lying right on the fault, and that the transition is only a few feet wide.
As promised (threatened?) here is some more on the geology.
The hike starts in the Swamp Creek valley on rocks of the Easton terrane, called the Darrington Phyllite. Goat Mountain, across the creek, is one big chunk of phyllite, and we’ve seen it often on other trails (Anderson Mountain…), but we don’t see much along this trail since it is covered by glacial deposits and a lot of vegetation. Most of the gold mined in the Mount Baker area was found in quartz seams in this formation.
As the trail rises through the woods it crosses onto the Chilliwack River terrane, a broad mixture of various volcanic, sedimentary and metamorphic rocks that is very widespread in the area (Herman Saddle, Canyon Ridge, Sauk Mountain…) but the forest hides the transition. The first really noticeable rocks don’t appear until you are on the section of the trail that contours around to the north and west from the Gold Run Pass / Tomyhoi Lake trail toward Yellow Aster Butte and Mount Tomyhoi, and, at first, these are the Chilliwack rocks.
Now the Easton and the Chilliwack terranes are very nice, I’m sure, and very important geologically, but it is the third terrane that has always interested me most. It is called the Bell Pass Melange, named for a seldom-visited pass at the head of Bell Creek, a tributary of the South Fork of the Nooksack. That pass is a saddle in the ridge that joins Mount Baker to the Twin Sisters and separates the South Fork from the Middle Fork. (That really has nothing to do with today’s story, but I put it in to emphasize that these various terranes are all jumbled up and crop out all over the place. It is a very, very complex system.) The geologists call it a “melange” (French for “mixture”) probably hoping that it will sound sophisticated and most people won’t notice that they really don’t completely understand what’s going on here.
There are several very different types of rocks within the melange, and each of them is strange and extraordinary. The most prominent local exposure of this terrane is the Twin Sisters Range, which is one great big chunk of ultramafic rock called dunite which is composed entirely of one mineral, olivine (which is unusual in itself), and began it existence as part of the earth’s deep mantle, rather than the thin, crispy crust we live on. Somehow it got attached to a piece of crust that got turned upside down in some violent tectonic interaction.
Yellow Aster Complex
Yellow Aster Complex
Another is the Yellow Aster Complex (note the use of another general purpose noun to cover a lack of full understanding). I went prospecting for this one a few weeks ago on my trip to the Baker Lake region. It is very old and is sometimes called the Yellow Aster Gneiss, since it is rather coarse grained and has undergone metamorphism, but it is cut with other things, and is truly a complex within the melange. I’ll call it “Yac”.
It was named for this general area, probably for the area west of the butte where the chain of small lakes is and where a good deal of prospecting and perhaps mining went on at one time. That area is entirely underlain by the Yac; but the part of Yellow Aster Butte that the trail leads to is actually built of the third rock species.
Bell Pass Melange
Bell Pass Melange
My favorite is also called, confusingly, the Bell Pass Melange (Bpm). So it is a melange within the melange. (From now on I will use the term “melange” and “Bpm” to refer to this particular rock, rather than to the whole terrane. Why do people do things like that?) It is a truly gnarly and mixed up mess. It is dark gray to black overall, but with bright white quartz intrusions, folded and layered parts that almost look like petrified wood, cracks, crevices and what-all. When you hike that last steep slope up to the “top” of Yellow Aster Butte, this is the stuff you are stepping on.
As I mentioned above, it seems to weather rather evenly, so it tends to hide from view under vegetation, but substantial outcrops can also be found in the Middle Fork area (near Elbow Lake and, strangely enough, near Bell Pass) and near the west end of Canyon Ridge.
These pictures are of samples I gather near Schrieber’s Meadow (Yac) and the Middle Fork (Bpm). The last one is of a small piece, also from the Middle Fork, that I sliced and partially polished in my lapidary class last year. The teacher wouldn’t let me cut it in the big slabbing saw because it looked like it would shatter and jam up the works. You can see many small cracks and rusty spots and near the upper left you can see some crystals forming a small geode. It is really strange stuff.