Winchester Mountain trail-side geology

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Fig 1. The geologic highlight of the Winchester Mountain trail, other than the stupendous views into the complex geology of the North Cascades, is the trail climbing along a yellow dike cutting cutting across the basement rocks. This view looks east from the ridge crest at the top of the dike. OPEN ANY IMAGE FOR A CLOSER LOOK.

Winchester Mountain rises steeply above Twin Lakes in the Mount Baker Wilderness northeast of Mount Baker. A restored fire lookout graces the summit. The hike is 2 miles and 1300 vertical, but the access includes a very rough road (often all wheel drive at minimum). Nevertheless, the hike is popular. The attraction, other than the fire lookout, is the stupendous view of the northern reaches of the North Cascades.

Geology tools for this hike- you may want your handlens; no need for a hammer, plenty of rocks to look at without.


Drive the Mount Baker Highway for 42 miles east of I-5 in Bellingham. Turn left on the gravel Swamp Creek Road (Forest Service Road 3065) immediately past the Department of Transportation maintenance sheds. It is 4.5 miles to the Yellow Aster Butte trailhead (trail side geology guide in the works). The road gets really rough and technical beyond here. Don’t count on going much beyond the YAB trailhead unless you have high clearance and all wheel drive. Be prepared to walk 2.5 miles up the road from here to Twin Lakes. Maybe some good Samaritan will offer you a lift. I made it fine in my Subaru Crosstrek. It was fun. You probably won’t have much fun if you don’t like rough roads or know how to get your car over washouts and high rocks. I highly recommend checking for road reports before trying to drive up to the trailhead. Washington Trails Association has frequent visitor updates. The Forest Service has an official report for Road 3065 as well.

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Fig 2. Trail map [at the trail head]. Distances in miles, north is up. The intrusive dike is discussed below. It’s an unmistakable landmark 300 feet below the lookout and it means you’re getting closer.

The trail begins in the narrow isthmus between the two lakes. It is mostly in the open, with ever-widening views. It entire shows very clearly on Google Earth.


This whole area is a helter-skelter geologic mishmash of unrelated rocks faulted against and over each other. Fortunately, the rocks along the Winchester Trail are relatively simple and with one significant exception, all are mapped as a formation of similar rocks within a larger geologic unit called the Chilliwack Group. The Chilliwack Group of formations are Pacific Ocean seafloor rocks, once part of the Farallon Plate, that were accreted to the western margin of North America during subduction of eastern portion of the Pacific seafloor. The Farallon Plate is now long gone. Subduction is now swallowing the Juan de Fuca Plate off our coast- but let’s not get bogged down in plate tectonics here. 

Fossil ages in Chilliwack sedimentary rocks have a wide range from the Silurian to the Permian Periods- roughly 430 to 240 million years old. All these rocks were accreted to North America as a part of a large terrane called ‘Wrangellia’ long after that, around 100 million years ago. (I discuss accretion of Wrangellia and other terranes onto the margin of North America in the first section of my book, Geology Underfoot in Western Washington; see the E Pluribus Unum’ section. ‘Order the book from the publisher if its not in your local book store.) The sub-unit of the Chilliwack Group along the trail are volcanic rocks, likely part of an island arc erupted onto the ancient seafloor.20220911_124159 labeled

Fig 3. A small portion of the geologic map in Tabor and others, 2003 (see references below). North is up. the grid squares are 1 mile on a side. The Winchester trail travels through the brown geologic unit between Twin Lakes and the lookout. This is the Permian to Devonian aged Chilliwack volcanics, at one time part of an oceanic plate accreted to the western margin of North America. I added the red line to locate the dike crossed by the trail. The red circle on the Winchester Fault is discussed below. Another map unit of interest is the tan area right of center labeled ‘Tvbb’- this is the ‘Tertiary volcanics of the Big Bosom Buttes, a caldera that is only very generally mapped and largely unstudied. A photo from the lookout is included below. Masters thesis anyone? The rest of this geologic map is very complicated, and deserves its own page on this website to try explain some of the geologic symbols. Watch for a notice once I have tackled this.

Greenstone refers to a volcanic rock that has been metamorphosed by pressure and percolation of hot water after burial in the seafloor crust and subduction.  I write about some younger greenstone in the area in my page on the Wells Creek Formation, down in the North Fork Nooksack near Nooksack Falls. There’s a quarry in fresher-looking greenstone.  Metamorphism changes the original minerals, such as plagioclase feldspar, olivine,  and pyroxene, into green minerals- typically epidote, chlorite, or actinolite.

The most distinctive rock you’ll see is a very prominent pale yellowish dike that is not shown on the geologic map; it’s just too localized at the small scale of the mapping. I don’t know the age of this intrusion, but it is far younger than the Chilliwack volcanics and may date to an early period during the uplift of the Cascade range. Regardless, it is very cool and the geologic highlight of the trip, for me.


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Fig 4. The craggy summit of Winchester rises above as you enter the large meadow with pale blocks.

You’ll soon come to the junction with the High Pass trail branching sharply to the right; this is your first landmark. In the big meadow opening you’ll see the summit crag and will come across the first rock you’ll see, tumbled blocks from the cliffs above. The rocks in this talus deposit aren’t exactly inspiring,and you may be hard pressed to see anything interesting in them. Look at a number of them. A couple I noticed are clearly made up of fragments of other rocks- so they have a texture called breccia (Italian for ‘broken’); I can’t tell just what the fragments are, but this is probably rock derived from fragmented lava flows, perhaps submarine.

Figs 5 and 6. The greenstone block on the left has angular fragments, so leads me to think this block is made up of fragmented lava. The brown one on the right has 1-2″ rounded pebbles. I don’t expect rocks to get rounded on the sea floor, so I’ll guess the pebbles were eroded on land, either by a stream or on the beach, before cemented and lithified to form a conglomerate.

Now we’ll digress a little and look closely at another block a bit further up the trail. Many blocks here and indeed anywhere can change appearance over a space of a few feet or even inches. This has nothing to do with the source or origin of the rock, rather it’s an example of how metamorphism and weathering and oxidation can affect the appearance of a rock. There are many examples like this, so just keep an eye out.

Figs 7, 8 and 9. These are all the same outcrop, and all three are only a couple feet apart. Open each image for a larger view.

The dark greenish-gray one at left  looks like tuffaceous breccia- zoom way in to see some of the angular fragments just to the right of my patented ‘fish scale’. Tuff refers to very fine grain volcanic materal- ash or very finely broken fragments. Just out of view a foot or so below is the area shown at upper right. Oxidation has deposited an iron-bearing stain on the surface of the rock, perhaps along a fracture that admitted water into the rock long ago- I don’t think this is recent, or it would be pervasive over the entire rock surface. Above the stain is a cracked up white area; desiccated dark moss grows in some of the cracks. The white surface may be a result of weathering that chemically altered the surface of the rock to  pale-colored clay minerals. I really don’t know, some lab work would be needed. Finally, the rock at lower right is a 1 foot to the left and a bit fresher looking and the green color is evident.

OK, lets keep hiking. At one point as you head west it look like the trail is finally reach a big dark outcrop but then switch backs away, only to tease us with another turn and nearly get there. But, no, not quite, it switches right yet again and heads away- only to come to a fabulous viewpoint on the southeast ridge of Winchester. Here you get the first really great views east into the Cascades.

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Fig. 10. View a bit north of east from the southeast rib of Winchester Mountain. Geology in this view is dominated by the Chilliwack Composite Batholith composed of multiple intrusions of magma (plutons) that cooled and crystalized in the crust over the past 25 million years or so. The youngest intrusion is only a couple million years ago, Read descriptions of some of these plutons in Tabor and all, 2003. The Pleiades are just south of the border with Canada; Slesse (6 miles away) and Rexford are in Canada. Winchester Creek is at right flowing east from Twin Lakes toward the deep hideaway of trail-less Silesia Creek valley. True wilderness, that. In fact, I don’t believe there are any formal trails anywhere in this view; a few climbers tracks maybe, that’s it.

After you soak in the view ( I hope you have one!) on in Fig. 10, continue on. The trail runs westward in a quarter-mile long traverse along the 6200′ contour (see the map above) toward the southwest ridge of Winchester. Along the way the trail goes through a greenish gray blocky outcrop of greenstone (but more gray than green). This was massive lava, rather than breccia or fine-grained tuff. Photos below.

Figs 11 and 12. The trail goes right by some decent looking, identifiable rock. The photo on the right shows speckled white mineral crystals. These were originally shiny, largely transparent plagioclase feldspar. But the crystals now have a ‘soapy’ appearance (think tiny bars of Ivory) because the calcium component in the plagioclase is altered even by relative low metamorphic pressure to form an assemblage of replacement minerals like calcite, epidote, and chlorite. I doubt you’d find many fresh shiny crystals.

In another couple hundred yards, you reach the coolest geology along the Winchester Trail. You can see it coming up- a band of yellowish rock cutting across darker rock at the end of the long western traverse toward the southwest ridge.

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Fig. 13. Nearing the dike, the yellow rock along the trail before the ridge crest.

The trail climbs along the dike and you might want to be a bit more careful here. You’ll for sure want to take some time to look at the contact between the dike and the rocks it intrudes. The magma in the dike invaded a fracture in the host rock. You may know that I really like igneous rocks in particular so this outcrop gives me particular pleasure.

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Fig. 14. The dike is about 2 meters wide. Ooops, forgot to provide something for scale. Sorry, broke a cardinal rule of geophotography.

Figs. 15 and 16. Here are two views of the surface of the dike. On the left is the weathered surface, a uniform tan. You can see plagioclase feldspar crystals if you click to enlarge. A more-freshly exposed surface is on the right, darker and the plagioclase is more obvious. I think I also see some dark pyroxene. The rock in this dike is fresher looking than the

The dike is porphyritic igneous rock- that’s a rock texture featuring conspicuous large crystals in a finer grained matrix. It may be andesite; it’s hard to say what the original composition of the magma was with any certainty, as it has been altered since it intruded the host rock. But it is not metamorphosed and the dike itself has not been deformed as it would have been if it was subducted along with the Chilliwack rocks. It’s much younger, in any case.

Figs. 17 and 18. Blow up these images to closely examine the knife-sharp contact between the dark Chilliwack volcanics and the dike. In Fig. 17, left, there is a subtle fracture in the dike (look near my hat) running parallel to the contact in the photo on the left, and the 2-inch-wide change in color and appearance of the dike rock in Fig 18. Read on.

The sharp contact and the porphyritic texture of the dike rock suggest to me that the intrusion occurred pretty high in the crust- the host rock was cold and the magma cooled quickly so larger crystals did not have time to grow. In such situations dikes develop a chilled margin, where the hot rock very quickly solidifies in contact with the host rock. The margins of this dike may preserve that indirectly. In Fig. 17, a fracture runs parallel to the contact, just inside the dike. I have seen fractures like this in much fresher, younger dikes. They develop between the very fine grained, glassy chilled margin and the interior of the dike, which cooled more slowly. Fig. 18 shows the color change near the outer margin of the dike. If this is a chilled margin, it has discolored due to weathering- not unexpected in rock that is finer grained, even glassy, compared to the adjacent slower-cooled magma in the body of the dike.

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Fig. 19. Another similar dike cuts the rock knob to your left. This one runs at about a right angle to the one the trail crosses.

I’m not sorry to belabor this business about the dike because it raises good field geology questions. One  is “why does this dike suddenly appear on the trail?”  And another, “what happens to it at the ridge crest just ahead?” The geologic map doesn’t help here, it isn’t detailed enough to show the dike- I drew it in on Fig. 3. For the first, note that the trail traverses a steep meadow for the hundred yards or more before it reaches the dike and the outcrop of rock it intrudes. The rock itself is not exposed in this section, eroded away including the dike. Some portion of it may be there, but is buried in scree and soil. See Figs 1 and 13 to see what I mean.  As to the second question, it looks like the Winchester Fault mapped by Tabor and others runs right along this ridge crest (go back to Fig 3. and blow it up: the fault is labeled near the bottom edge of the map). The fault is shown as a north-south black line for over three miles. On the north, it ends under glacial ice. If you look on the full-sized geologic map in Tabor and others (2003) you’ll see the authors suspect (they use a dashed line to indicate this) that the fault continues south through Goat Mountain towards Mount Sefrit. They show it running through the two peaks of Goad Mountain, and use gullies, as shown by contour lines, to trace the fault- faulted rocks are weak and subject to erosion. The red circle I drew on Fig 3 points out a mapping convention used for faults. The small dot-on-a-dash symbol attached to the fault indicates that relative fault movement was down to the west (left) and/or up on the east (right). So, the west end of the dike was dropped downward by ancient movement on the Winchester fault. It might appear again as an outcrop somewhere below in rocks on the west side of the fault, or might be eroded or covered with landslide deposits or brush.

If you like dikes, I describe another one at Marrowstone Island. That one is dark basalt and right on the beach. Its pretty cool, you should go for a beach walk and check it out.

image3 0.50 markFig. 20. Reach the southwest ridge of Winchester Mountain at the end of the dike exposure,  right on the mapped location of the fault, and begin the final 300′ climb to the Winchester Lookout, restored and maintained by the Mount Baker Club. Their website has a lot of info on the lookout’s history and the restoration. It was built in 1932 by CCC crews out of the camp at Glacier.

We’ll close our geology field trip with some of the view. Mount Baker dominates to the west. The twin East and West Peaks of Goat Mountain are above Twin Lakes to the south.

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Fig. 21. Looking north toward Mount Larrabee. To the right are the craggy peaks of the Pleiades. The steep spire on the left is American Border; the international boundary separates it from Canadian Border Peak just beyond.

To the north is 7865′ Mount Larrabee, a tottering heap of eroded Yellow Aster gneiss. More interesting geologically is the view to the east. The north-south lumpy ridge just to the east is formally named ‘Skagit Range’ ; it is also known as ‘Silesia Ridge’ for the major drainage on either flank. A more widely used, but still informal name for the highest peaks is ‘Big Bosom Buttes’. They are mapped as an Oligocene-aged caldera and shown on the geologic map (Fig 3.) as a brown unit labeled Tvbb- Tertiary volcanic rocks of Big Bosom Buttes’. Other than reconnaissance mapping (using a helicopter in the 1960s) these 2500-foot-thick tuffs and breccias are not studied in detail. The caldera as mapped (or what is left of it) is around 3 miles in length and only a mile wide. But 20 million-plus years of erosion accounts for a lot of missing rock. Studying these rocks and the structure around them could make a great masters thesis for someone with rugged cross-country travel and scrambling skills.image0 mark label

And of course the view down to Twin Lakes, 1300′ below. Can you see your car? I can.


Tabor, R., Haugerud, R., Hildreth, W., and Brown, E., 2003, Geologic map of the Mount Baker 30- by 60-minute quadrangle, Washington: Geologic Investigations Series I-2660. US Geological Survey. The map, cross sections and explanatory pamphlet are available online.

Tabor, R., and Haugerud, R., 1999, Geology of the North Cascades, A Mountain Mosaic. 144 pages. Mountaineer Books, Seattle. If you don’t have this book, it is an indispensable peoples’ geology guide. It has nice sketches, trail descriptions, and a practical intro to our complex geology.

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