February 23, 2010

Mounts Erie and Sugarloaf, and the Fidalgo Ophiolite

Filed under: Geology, Hiking — geezerwriter @ 10:29 pm

Last Thursday we were scheduled to hike to Mount Erie and Sugarloaf Mountain on Fidalgo Island, near Anacortes. For a change I remembered to check the geologic map and found that those “mountains” (merely 1300′ and 1100′ high, respectively) are part of a structure called the “Fidalgo ophiolite”. That set me to searching and reading up on ophiolites, and yes, you will be learning about them, too! (Fair warning.)

An ophiolite is not a kind of rock but a series or sequence of related rocks that pops up in a number of places around the world. While the Fidalgo ophiolite is certainly not unique, it belongs to a pretty rare family. An ophiolite sequence can consist of sedimentary rocks, various kinds of volcanic rocks, an intrusive igneous rock called gabbro, all piled on stuff like serpentine and dunite which are actually part of the earth’s mantle. We’ve run into most of those types of rocks (except the gabbro) many times on our hikes in the North Cascades, and most are pretty nondescript, but I was all fired up to see if I could find any of this stuff in real life. Especially the gabbro – I’d learned of it over fifty years ago when I was a freshman geology major at Notre Dame and seen pictures in books, but I’d never encountered it in the wild. [The big black cliffs on the flank of Mount Sefrit that tower over Ruth Creek and the Hannegan Pass trail are reportedly made of gabbro; I’ve intended for years to bushwhack across the creek to the base of the cliffs to snag a sample but haven’t done so, yet.]

The Olympics over Admiralty Inlet

We had another beautiful day for our hike – sunny and chilly as we started out, but quickly warming into the fifties. The hike runs mostly through the woods, with no grand views. We weren’t going terribly fast, and I wasn’t leading, so I could pay some attention to the rocks as we went.

Another reason that I was glad not to be leading: I didn’t know where we were most of the time! The area is a virtual rabbit warren of intersecting and overlapping trails. It is all part of the ACFL, the Anacortes Community Forest Lands, which was acquired and retired from active forestry (i.e., clear-cutting) by the fine folks of Anacortes, who had the generosity and foresight to see that forest lands could have some value beyond that of the potential toilet paper in the tree trunks. It is a wonderful resource for which we should all be grateful, and a hiker might ask, “Can you ever have too many trails?”

I think the answer may be “Yes.” We had a pretty big group (18) so it was inevitable that, despite our modest pace, we would get a bit spread out; there were so many trail intersections that we had to be almost constantly on guard to see that the slower hikers came the right way.

Mount Baker

But still I could keep my eyes on the rocks much of the time. As usual, most of the lower-lying places were filled in with glacial till from the recent (geologically speaking) continental glaciers, and some parts of the trail were old roadways that had been built up with rock that might have been trucked in from anywhere, so the only rocks worth looking at, from my ophiolite-centric point of view, were bedrock exposures. And as usual, again, when there was a big enough chunk to be pretty sure that it had been around for more than a few thousand years, it was likely to be covered with enough moss and lichen to turn any kind of rock into a bland greenish-gray lump.

Actually, though, that’s what I was looking for. According to the geologic map, we would be moving across various kinds of volcanic (i.e., lava) rocks, which are themselves pretty pretty dull and nondescript. Now volcanos are certainly interesting, even exciting sometimes, in themselves (e.g. Mount Saint Helens, ca. 1980) and lava flow sequences can tell interesting stories, but the individual chunks of rocks are kind of boring. They were extruded onto the surface where they cooled very quickly, not allowing time for the individual minerals to gather together into the pretty little crystals that you see in intrusive rocks like granite. Has anyone ever tried to sell you a lovely basalt countertop for your kitchen remodel? I didn’t think so.

Gabbro makes a good bench

The map had promised, however, that if we went far enough south we would to get to the intrusive part of the ophiolitic sequence: that elusive gabbro. But would I find it before we got the the very steep cliff that drops off the south side of Mount Erie and attracts to hang gliders, parasailors and rock climbers to the area? It made sense that I would: the developed crystal structure of gabbro, like other intrusive igneous, gives it great hardness and durability – if anything could survive the scraping and gouging of the glaciers, this would be it. So as we neared the summit I was taking my time and peering closely at every outcrop. I saw nothing that would tell me that the rock wasn’t gabbro (it was pretty dark colored, but by no means black; it didn’t show any flat spots, like slate or schist) but the lichens continued to homogenize everything.

But just as I almost caught up to others at the lunch spot on the summit, I saw a spot that looked different. It was right near the edge of paved trail, and perhaps someone had broken off a piece when building the forms. (I should mention that there is also a road that leads up to the summit, and a big communications facility of some sort.) On closer inspection, I could see a dark, sort of greenish rock with a definite crystal structure – just what I’d been looking for! I stupidly forgot to take a picture of it – the nearest thing I have is one of Cindy and Janet sitting on the gabbro and gazing at the view.

Did I mention that it was a beautiful day? I had never seen the Olympic Mountains so clearly without actually being on the Olympic peninsula with them. But the big surprises were the views of Mount Rainier and Glacier Peak. The latter is always a special treat, since it is so far back toward the Cascade Crest, and so far from what passes for civilization, that you hardly ever get a good look at it. Indeed, the summit of Mount Erie is, to my knowledge, the only place anywhere where you can drive and get a good look at Glacier without a significant amount of hiking.

Glacier Peak and Whitehorse Mountain

On the way back we took a slightly different route, visiting the summit of Sugarloaf Mountain (if I dare use as grand a word as “summit” for the top of this hill). As you can see from the picture below, the exposed bedrock is a bunch of lumps. [Now, I am only talking about the rocks, to be sure.] They are typically lichen-encrusted and I wasn’t able to find any pieces with fresh faces [Really, just the rocks!] so I can’t say anything about their composition, but I’m betting that these are the pillow basalts that also a typical component of an ophiolite. That made it a very good day for me, geologically speaking.

Pillow basalt(?) on Sugarloaf (with hikers)

On the way back we ran slightly afoul of the “too many trail” phenomenon, but as always, Pat led us back to our cars. I really don’t know how he does it in places like this and in the Chuckanuts – I would be in deep doo-doo without my GPS


And if you think that was a lot of geology talk, you ain’t seen nothing yet. Once I got started writing about ophiolites, the professor-that-refuses-to-die inside me took over and just wouldn’t shut up. Figuring that not everyone is enthralled by the niceties of orogeny and geologic history, I decided to split most of the ophiolite talk off into a separate posting with a clear “reader discretion is advised” warning.



  1. Nice description, Al! Now I’m off to get educated at the second posting. And nice pictures, too!

    Comment by DJan — February 24, 2010 @ 7:30 am

  2. […] […]

    Pingback by visiting Anacortes/LaConner - Washington (WA) - City-Data Forum — September 16, 2010 @ 5:15 pm

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