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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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Geology Guide to the Blue Lake Trail (Washington Pass)

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On the trail to Blue Lake.

Blue Lake is a gem, nestled in a small rock-bounded hanging valley above Washington Pass on Washington State Highway 20. It is accessed by Blue Lake Trail #314 in the Okanogan-Wenatchee National Forest.

Geologic highlights: True granite, unusual in the North Cascades; granite boulders weathering to coarse sand (grus); a cirque lake; an older granitic rock intruded by the granite; and the screen of rocks separating them. A hand lens will help you see the mineral crystals in the granitic rocks. Oh, and grand alpine scenery.

Getting there: The trailhead is about a mile west of Washington Pass on Highway 20. It is 100 miles (160 km) east of I-5 at Burlington, up the Skagit River. It is 30 miles (48 km) west of Winthrop. The trail head elevation is 5,477 feet (1.669 m). The hike is 4.4 miles (7.7 km) round trip, and the elevation gain to Blue Lake is 1,050 feet (320 m).

This hike is very popular. Highway shoulders may be lined with hikers cars. I recommend you go midweek if you can. Fall is particularly nice because the larches turn bright yellow before losing their needles. NO CAMPING AT BLUE LAKE.

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Annotated Google Earth image. North is up. The trail is pale blue. Three field trip stops are marked. The red line is the approximate contact between the Golden Horn Batholith (above) and the Black Peak Batholith.

About batholiths and plutonic rocks: A batholith (Greek bathos, depth + lithos, rock) is a mass of intrusive igneous rock that exceeds 100 kilometers² (40 mi²) in area. The rock originated as magma deep in the crust, usually at the boundary of the crust and the underlying mantle. Because the hot rock is buoyant, it rises slowly upwards through the crustal rocks. The mechanism by which such a huge volume of magma rises is controversial. The ‘Washington Pass’ chapter in Geology Underfoot in Western Washington discusses the various theories for how this happens. This field guide won’t get into all that- get the book. (Ahem. Subtle hint by the author). The magma eventually stalls and ceases to rise as a mass, although a small part of it may reach the surface via fractures and erupt as a volcano. The  cools to solid rock over hundreds of thousands or even millions of years. Eventually, erosion removes enough of the overlying rock to expose part of the batholith. Batholiths consist of granitic rocks, whether granite, diorite, granodiorite, tonalite, or some other rock type distinguished by the proportion of specific mineral crystals. The diagram shows how the proportion of three index minerals (quartz, alkali-feldspar, and plagioclase feldspar) are used to name rocks.

 

600QAPplutonic

This is complicated. Q=quartz, A=alkali feldspar, P=plagioclase feldspar. For a tutorial on using a ternary diagram, go here.

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Close view of Golden Horn granite. The pencil points at biotite mica, the most common mafic mineral. The clear crystals are quartz; the white is feldspar, mostly alkali feldspar; it is hard to differentiate the plagioclase feldspar without a hand lens.

The 49-million-year-old Golden Horn Batholith is a rare thing in the North Cascades. It is a true granite, containing recognizable amounts of all three of the index minerals, plus dark biotite mica and an unusual amphibole, arfvesonite. Most plutonic rocks in the range are relatively poor in quartz in and alkali feldspar, and rich in plagioclase, and are thus classified as granodiorite, tonalite, and monzonite of various flavors. The Golden Horn granite was emplaced into pre-existing rocks, which were either deformed and shoved aside or subsumed into the mass of magma. Some of these older rocks are sedimentary rocks, remnants of older seafloor accreted to the margin of North America, or are volcanic rocks associated with earlier phases of volcanic activity. Adjacent to the Golden Horn, to the west and south, is a much older magma body, the 91-million-year-old Black Peak Batholith. It is mainly salt-and-pepper speckled tonalite and granodiorite (see the ternary diagram) so very different from the Golden Horn. This rock dates to the time after a microcontinent, Wrangellia, was slowly accreted to the margin of North America, generating magma and uplifting a pre-Cascades mountain range. We won’t actually lay hands on it along the trail, but we’ll see cliffs of it above Blue Lake.

So, let’s start walking. At first the trail parallels the noisy highway. A short way beyond the first switchback to the right, note a talus of boulders. These have fallen and tumbled from the cliffs of Liberty Bell Mountain up the slope. There are two more switchbacks down in the forest, then begins a long curving traverse to the south and then southwest. Pass through a couple of open avalanche chutes, with outcrops of granite becoming more common. Be on the lookout for granite boulders that are weathering into coarse sand and pebbles consisting of mineral crystals. The weathering is caused by freeze-thaw. Water that percolates between the crystals expands when it freezes, and the crystals are loosened. Eventually the whole boulder breaks down into fragments.

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My pack marks a good spot to spot  ‘solid’ granite boulders weathering into sand. The UTM is (Zone 10) 0672417, 5375907. (NAD27).

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This boulder, just up the slope from my pack in the previous photo, is shedding crystals onto the ground at its base.

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Scale at right is millimeters. Dark crystals are amphibole (probably arfvedsonite); the pinkish orange is alkali feldspar; there are a few shiny squarish plagioclase feldspars , and quartz are the more irregular blocky crystals.

You are now on the home stretch to Blue Lake, contouring southwest. Granite crags rise above you to the east; you may spot rock climbing parties leaving the trail and heading upward toward the base of the spires. There will be great views from the lake. For now, watch for the ‘No Camping at Blue Lake’ sign. Some interesting and different rock outcrops begin a short way beyond the sign.

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This  loose block lies in the trail beyond the ‘No Camping’ sign. It is the first non-granite I spotted on the trail. It is intruded by pale dikes of Golden Horn (?) granite. No, the book is not still there. Buy your own.

The first of these is a gray rock cut by a couple of white dikes (photo above). I am not certain, but I’ll betcha these dikes are Golden Horn magma invading fractures in the host rock, which must therefore predate the arrival of the batholith. If you look closely, you will see that there are darker bits of something else in this rock. It isn’t clear here what those are, but I believe we’ll get some insights beyond the lake. In quick succession you’ll pass other rocks that are also ‘not granite’.  Some of these are fine grained, brittle and break along sharp-edged fractures. There are dikes of Golden Horn granite (again, without a chemical analysis, I can’t be certain) invading many of these small outcrops. If If I am right, all these rocks lie along the margin of the Golden Horn intrusion, and were likely baked to some extent by the huge body of magma invading the crust only a few meters away. Photos of some of these rocks follow. The Black Peak Batholith must have invaded these same rocks around 40 million years earlier, and lies just to the west. These remnant host rocks are therefore a thin ‘screen’ separating the two batholiths.

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This rock may be the same rock cut by the dikes; I think it has been deformed from the pressure of Golden Horn intrusion. It is at 0672114, 5375302. It appears to be cut by a few very thin dikes of Golden Horn.

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And finally, here is an outcrop with dark gray blocks of the same rock we saw closer to the sign, swimming in a whitish dike of Golden Horn. These rock fragments were broken off by the injection of the magma, and carried along in the dike.

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Here is another example of fine grained (meta-sedimentary?) host rock cut by a dike intruding an angular fracture. Surfaces of this outcrop are stained orange from oxidation of iron-bearing grains.

Cross a stream and come finally reach the lake, lying in a cirque. You will come to a large slab rising out of the blue green water (a good place for lunch). Views of the Liberty Bell group of spires are good from here. But the main geologic point here is the high walls bordering the lake to the south and west. These are granodiorite of the Black Peak

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Looking toward the head of the Blue Lake cirque, the rock knob on the left is Golden Horn granite. The cliffs to the right are Black Peak granodiorite. The contact between them runs through the notch. The contact runs under the lake and talus…

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…and continues to the north across the highway,, through the lowest notch between spiky Cutthroat Peak (Golden Horn, on the right) and the darker Whistler Mountain, left.

Batholith. They have shed lots of talus as aprons around the lake. If you pick your way along the west side of the lake into this talus field  you can examine fallen blocks of the Black Peak rock. I will address that at the end of the rest of the trail description. For now, continue upward now west of the lake for 100 yards or so, keeping to the right. Come out onto a ledge of rock with a steep drop to the north. Get down on your hands and knees and you will see that this rock contains 3-5 mm dark angular minerals  in a lighter gray15242 mark matrix. The dark minerals are more resistant to erosion so protrude from the matrix. This is almost certainly the same as the rock we saw cut by the dikes before we got to the lake, and here is the best exposure I found of the pre-batholith rocks in the ‘screen’. I don’t know what these minerals are for certain, but they look like cordierite, a mineral formed during contact metamorphism of rocks exposed to considerable heat from intrusions. Such rocks, heated by intrusions, are lumped into a rock type called ‘hornfels’.

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Here is the ‘Liberty Bell Group’ of crags above Blue Lake. Left to right: Liberty Bell (7720 ft / 2353 m); really pointy Concord Tower and Lexington Tower (both are 7560 ft / 2304 m); and the two Early Winters Spires- North (7760 ft / 2365 m) and South (7807 ft / 2380 m).

Now, if you haven’t taken the time, enjoy the view of the rock towers to the east. They are separated by joints running through the Golden Horn granite. These joints are fairly uniformly spaced fractures, generally running east-west, that developed as the rock as pressure was relieved on the rock mass as it was uplifted higher into the crust. Joints form parallel systems in at least one, and often multiple directions. The fractures are exploited by freeze-thaw erosion and result in isolated rock towers as erosion advances.

So finally, for those with the drive, go back to the lake shore

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a 10-cm block of Black Peak granodiorite or tonalite. I didn’t make a point count of the actual ratio of the minerals, so I’ll be cautious naming this particular specimen.

and pick your way into the cirque. You don’t have to go far to reach Black Peak talus. This rock is really different from the Golden Horn. Crack open a block, or find a freshly broken one. For one thing, it has virtually none of the pinkish-yellow alkali feldspar, so is simply black and white. The dark mineral is biotite, the white one is plagioclase feldspar. There is also some grayish, glassy looking quartz. The adventurous can continue all the way around the lake. Go for a swim if you dare. The fool in the photo did. Its only a little bit cold. OK, it’s REALLY cold!

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This guy must be some kinda nut.

 

 

 

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