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    Northwest Geology Field Trips, by Dave Tucker, is licensed under a Creative Commons Attribution-Noncommercial- Share Alike 3.0 United States License. You can use what you find here, repost it with attribution to the author, "remix" it for your own purposes, but may not use it with the intent of making money off of it.

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Excelsior Trail geology guide

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West along High Divide toward Church Mountain.

Excelsior Pass lies on the High Divide between Church Mountain and Winchester Peak north of the North Fork Nooksack River in the North Cascades. It can be reached by two trails. One is the ‘short way’, starting from the end of the Canyon Creek Road and passing the shallow Damfino Lakes. The other trail, much longer, and described here, begins on the Mount Baker Highway just east of Nooksack Falls. I will describe the Damfino Route in a separate post, coming up next.

Geologic highlights: Geology along this trail includes sparse outcrops of the Wells Creek Volcanics, the usual glacial till and erratics, and, from the pass on up to the 5712-foot (1741 m) high point just east, marine sedimentary rocks of the Nooksack Formation.

Getting there: The Excelsior Pass trailhead is 42.2 miles east of the I-5 — Mount Baker Highway (State Route 542) interchange in Bellingham. Seven miles beyond Glacier, watch for the right turn to Nooksack Falls. Continue on the highway 0.4 miles to the well-signed Excelsior Pass trailhead on the left. There is an outhouse and space for a half-dozen cars. The Northwest Forest Pass is required for parking.

The trail head elevation is 1830 feet (560 m). The hike to the pass is 9 miles (14.4 km) round trip. The trail switchbacks moderately steeply through old growth forest to Excelsior Pass at 5370 feet (1635 m) , gaining 3540 feet (1080 m). The trail is mainly in the forest, only breaking out into meadows at a tad over 5000′ (1525 m). But once you are out of the trees, the views get really good. At the pass, the trail meets the Damfino Lakes Trail coming up out of Canyon Creek to the north. From here you can hike east 1/2 mile (0.8 km) to the old lookout site on top of ‘Excelsior Pk’ (Pt 5712 on the Bearpaw Mtn. 7.5 minute topo). Flowers in the meadows can be outstanding.

Tabor map annotated

Generalized geology from Tabor and others, 2003. The Nooksack is redrawn in bright blue. Red: Excelsior Trail, to top of Point 5712′. Green: Damfino Trail. Jnw: Jurassic Wells Creek member; KJna: Cretaceous-Jurassic Nooksack shale and sandstone; KJnv- volcanic-rich Nooksack conglomerate and sandstone; Qag- alpine glacial deposits. Scale: each square is 1 mile

The Geology: The most detailed geologic map is in Tabor and others, 2003. Bedrock exposures are few, but the ones you see until you reach Excelsior Pass consist of greenstone of the Jurassic-aged Wells Creek Volcanic Member of the Nooksack Formation. OK, what does that mean? These 180 million-year-old volcanic rocks, largely andesite or dacite lava flows, are contemporaries of, and in places interfinger with, marine sedimentary rocks that characterize the Nooksack Formation. Because the Wells Creek rocks are volcanic, rather than sedimentary, they were classified as a distinctive sub-unit, or ‘member’ of the more widespread and thicker Nooksack Formation proper. Dark-colored iron and magnesium rich minerals in the lava were saturated with water as they were submerged and buried, and overtime were altered to green minerals such as epidote, chlorite, and actinolite. The rocks thus take on a (sometimes rather subtle) green color, hence the name.

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A fragment of greenstone from the Wells Creek Volcanics. Exposure to the atmosphere and water has produced the 1-cm-thick weathering rind on two sides of this bit of rock, but the greenish rock in the center is fresh greenstone.

The Wells Creek Volcanics are best described by Rowland Tabor and his co-authors in text pamphlet accompanying their Geologic Map of the Mount Baker 30- by 60-Minute Quadrangle, Washington (2003)¹ published by the USGS. I have discussed these rocks in an earlier page in my website  These rocks are primarily the remnants of a long-gone volcanic island arc that was out in the proto-Pacific somewhere. At least some of the lava erupted underwater, or was later submerged and buried by marine sediment, later to become shale, sandstone, and conglomerate. All these rocks were on the Farallon Plate. This huge slab of Earth’s crust was overridden by the westward-moving North American Plate and subducted. Parts of the Farallon Plate broke off and were attached to the margin of North America, a process known as ‘accretion’. If you want to learn more about these geologic processes in the North Cascades, check out the Introduction in my book, Geology Underfoot in Western Washington‘ and Miller and Cowan’s Roadside Geology of Washington.

Most trail cuts will reveal glacial till, the leavings of the great glacier flowing out of Central British Columbia that overwhelmed the Cascade ridges and filled the river valley with ice. The till was deposited as the ice thinned and left behind its load of rocks and dust.

Excelsior Pass has outcrops of Nooksack Formation conglomerate and breccia.

The Trail Guide: Within the first half mile you will pass a couple small cliffs of Wells Creek Volcanics. I found no fresh surfaces to examine. You will need a geologists hammer to smack off a chunk for a closer look- and a hand lens. It’s nothing to write home about.

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The hole angles into the ground at around 60 degrees. This is taken with a flash, the only way I could see anything at all down there. The log disappears into the gloom about 15 feet down. Weird huh?

At about 1/2 mile (800 m) note a very obvious hole on the right, maybe two feet across. It is deep, fading into darkness. I don’t know what this is. I puzzled over it with my geol-pal Scott a few years back. I don’t think it is man-made, though I suspect the thin log was placed there by humans. The surface around here, underneath the soil, seems to be loose boulders. Perhaps this place was undermined by ground water. I don’t know. Next time, I’ll hike up there with my skinny friend Miles (nearly 13) and stick him into it- with a rope and hardhat. I’ll have to promise him something really good first. Or maybe just promise to haul him back out.

A rib of resistant greenstone runs up the hillside east of the trail. It provides an opportunity to look at the volcanics. The first few left-hand switchbacks nearly reach it before turning away.

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The unnamed stream drains the large basin below Excelsior Pass. This is the largest rock exposure along the Excelsior Pass trail.

About a mile and half into your hike (1.6 km) reach a stream coursing down a steep slab of greenstone. The trail switches right here, away from the water, but it is worth noting for a welcome foot soak on the way down.

About 2 miles in (3.2 km) the trail makes a hard right at some log cribbing, a pretty distinctive

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The well-rounded granitic erratic, about a half meter across. Happy?

spot to this point. A couple hundred steps further, after a left hand switchback, watch for a white granitic erratic on the right, just as the trail reaches another switchback to the right. Since there are so many erratic fanatics who read this website (you know who you are!) I’ll point this one out. It is the most distinctive stone along the trail, and clear proof that this area was once glaciated- there are no granitic rocks for around 10 miles (16 km) to the east, and a bit more than that to the north. Glacial till makes up most of the trail cuts from here on.

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Watch for this stretch of trail to spot the Mazama ash at its end.

Finally, finally, finally, the trail emerges from the forest into scattered trees and meadow. Watch for a straight 50 foot (15 m) stretch of trail built up between some logs. At the top of this section is another curiosity: a small exposure of buff-colored very fine grained Mazama ‘Layer O’ volcanic ash erupted from the Crater Lake caldera in southern Oregon 7200 years ago, 420 miles (675 km) to the south. (Geologists  call it ‘Layer O’ in field notes due to the typical orange color, until the identity has been confirmed in the lab.) It wafted here in the huge eruption cloud that covered  a large swath of North America, many time larger than Mount Saint Helens 1980. Go ahead and take a pinch and roll it around in your fingers. Taste an itty bit (yes, this is an actual geologic field method). You’ll find that the ash is homogeneous, and barely crunches between your teeth, an indication of just how fine grained it is- almost clay. (Don’t worry, there is a stream just ahead). To really verify this is the Mazama Layer O ash, we would need to run chemistry on a microscopic glass shard, and ideally get a carbon date on the soil above and below. But we won’tP1040885 50 mark

Trailside exposure of Layer O. The trowel is 11 inches (27 cm) long. The contrasting dark stuff is soil from above.

bother. I am pretty certain this Layer O. It is very common in the North Cascades. Undisturbed Layer O in these parts might be 1-2 cm thick. What is curious is the nearly half meter thickness, and its limited lateral extent. Within only a few meters in either direction, there is no trace of it. The ash was concentrated right here shortly after it fell out of the eruption cloud, washed off the nearby surface, and accumulated in a hollow by rain or snow melt. This is fortunate for us, as this particular exposure of the Mazama ash is easy to spot and easy to examine, so far from its source.

Layer O distribution map

Distribution of Layer O from Crater lake caldera.

Layer O distribution key

I hoped to spot another distinctive, and local, volcanic ash layer above Layer O, the 6800 year-old BA ash from Mount Baker. Alas, no, not along this trail that I could see. (But there is a fine exposure of both these volcanic ashes in a meadow north of Excelsior Pass, along the Damfino Trail- see that description.)

Speaking of which, if you haven’t turned around yet, now is the time. P1040885b 50 mark

Admittedly, the geology hasn’t been stellar to this point, and you probably are pretty tired from the hump up the hill through the woods. But this is what you came for, right? And there is more to come. The trail makes a final traverse through wild-flower heaven (if you are there in July) to Excelsior Pass. “Excelsior“- it’s Latin for “ever upward”. But, no more. You are there….

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The final leg through floribunda to Excelsior Pass, in the trees ahead.

…unless you choose to continue east along the High Divide Trail to the top of Pt 5712, ‘Excelsior Peak’. Which we will in just a bit.

Before you do, take a look at the outcrops in the trail at and around the pass. We are at the transition between the Wells Creek Volcanic Member and the sedimentary rocks of the Nooksack Formation. Like I said, there is no hard and fast dividing line between the two. The rock here is breccia, meaning there are angular rock fragments, or clasts in the surrounding sediment. These have been shown to be fragments of andesitic or dacitic lava, almost certainly derived from the Wells Creek Volcanics, tumbled onto the seafloor and mixed with finer grained sediment- perhaps a landslide or slump off a steep bluff of lava, at one time fairly near where these rocks were deposited.

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Nooksack Formation at Excelsior Pass. Breccia has angular clasts. Conglomerate has rounded clasts. The first is indicative of a short travel distance between the source of the clasts and the deposit, not enough to round off edges. The latter indicates further transport, sufficient to round clasts.

OK, now let’s head up to ‘Excelsior Peak’. From the pass, take the trail to the east [that’s to the right, signed ‘High Divide’] and stay right at the fork just a few steps away. Traverse meadows. Note that the trail tread is small gray chips, characteristic of Nooksack Formation shale.  In a few hundred yards pass a prominent bedrock outcrop, the best seen on the whole trail. It is  well-bedded Nooksack shale remnant of an ancient seafloor. Rowland Tabor reported radiolarian fossils of probable Jurassic age here. They are microscopic so don’t get too excited! The layers are tilted steeply, the consequence of subduction, uplift, and faulting; they dip 65° to the north (see the ‘Chuckanut Road’ field excursion for more on dipping rock layers).

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300 yards east of Excelsior Pass, these trailside Nooksack Formation layers dip 65° north (to the left, more or less) . My friend Grace holds a flower for scale.

Round a corner and reach the crest of High Divide east of Excelsior Peak, then make a steep trudge to the summit, site of a long-gone fire lookout (1935-1968, then demolished). What a view! You are sitting on a hump of eroded Nooksack Formation; a fantastic 360° vista- west along High Divide to Church Mountain and Bearpaw Butte, north into Canada, (the prominent gray dome is Mount McGuire, a bare limestone peak of the Chilliwack Formation); east to peaks innumerable; and, of course Mount Baker, less than 12 miles (18.5 km) to the south. This is why people hike up here- not principally for the geology, I will admit.

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West toward Church Mountain and the craggy Bearpaw…

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North 6.5 miles (11 km) to Mount McGuire and the Cheam Range between the Chilliwack and Fraser River…

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East into the heart of the North Cascades…

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…and south.

References:

¹Tabor, 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.

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