Baker River Trail Geology. Green, green, green: rocks,
moss, water.
By Dave Tucker December 3, 2012
Baker River’s braided channel just above Lake Creek. Click to enlarge any image.
Baker River drains the wilderness heart of the North Cascades east of Mount Shuksan. The trail
ascends the narrow river bottom from the head of Baker Lake Reservoir 2.5 miles to the banks of
Sulphide Creek. It is a study in greens: the bedrock is greenschist, the trees are coated in glowing
green moss, and pools along the river are filled with green water. In fine weather, there are
mountain views. Do it in the murk and rain of early December, as I did with John Scurlock, and
you’ll perforce focus mostly on things close at hand. While the round trip is only 5 miles, there is
enough diverse geology to take up at least half a day.
Highlights: green (I mean really green!) Shuksan greenschist, river erosion and aggradation,
alluvial fans, rock slides, an interesting variety of rocks in river bars.
Baker River trail (red dashes) begins at the head of Baker Reservoir. Sheeseh, even the map is
green!
Getting there: The paved Baker River Road branches north off State Highway 20 16.5 miles east
of Sedro Woolley. The trailhead is at the end of the Baker Lake Road, 26 miles from Hwy 20
and three miles beyond the end of the pavement. If you are westbound on Highway 20, Baker
Lake Road is 6 miles west of Concrete. Alternatively, from Concrete take the Burpee Hill Road
and reach the Baker Lake Road in 4 miles (6.6 km). To do this, turn north by the old cement
storage silos on Superior Avenue at the west edge of Concrete, then turn left on Burpee Hill
Road at the T-intersection in a quarter mile. If coming from the west, head east up Highway 20
from Sedro Woolley. Turn north (left) on Baker Lake Road and reach the trailhead in 19.8 miles.
Trailhead elevation is 750 feet (~230 m). There is an outhouse at the trailhead. The Northwest
Forest Pass is required. The trail is moderate, with only a few short ups and downs.
Topographic map: Mount Shuksan 7.5 minute sheet covers most of the trail, beginning just south
of the Baker River footbridge, which is just north of Blum Creek.
Geologic map: Tabor, R.W., Haugerud, R.A., Hildreth, W., and Brown, E.H., 2003, Geologic
Map of the Mount Baker 30- x 60-Minute Quadrangle, Washington. Geologic Investigation
Series I-2660. US Geologic Survey. Online: http://pubs.usgs.gov/imap/i2660/
Geology guide to Baker River Trail
Shuksan greenschist nearly covered with plants. Note layering from metamorphic foliation of
minerals.
The trail sets off on an old road, and soon reaches the shore of the Baker River. Glacial ice came
this way from the BC interior at least six times over the past million years or so; ice abandoned
this valley some time around 12,000 years ago. More recent rock slide rubble lies on the glacial
sediment, along with alluvial sediment dropped by the Baker River as it reestablished itself in
post-glacial times. There is little bedrock along the trail, though there are many large blocks
fallen from the valley walls. All the bedrock is in the 3/4 mile or so. Generally it is covered in
moss, ferns, algae, and lichen, all very green. In the woods, even seemingly bare surfaces have a
thin coating of lichens, so it is pretty hard to examine actual rock surfaces.
Foliation is nearly planar in this exposure on a boulder just above the big bridge.
Shuksan Greenschist
Some low, water-smoothed outcrops in the river bed afford the best place to see bedrock. About
100 feet before you reach the big footbridge at the junction with the Baker Lake Trail, ½ mile
(800 m) from the trailhead, walk out to the riverside and head for the bridge footings. Before you
get there, you’ll see the slabs. The rock is Shuksan Greenschist, and is metamorphosed basaltic
lava. The parental Shuksan lava erupted onto the floor of the Panthalassa Ocean around 160
million years ago. The Panthalassa (Greek, ‘all ocean’; sometimes called the ‘paleo-Pacific’) was
the global ocean surrounding the Pangaea supercontinent. The supercontinent began to break up
at about the time the lava was erupting. The North American plate began its westward journey
(continuing today) and the western margin of the continental plate swept up small bits of seafloor
and volcanic islands during subduction. Many different terranes were thus accreted to the outer
edge of the continent, and the Shuksan is part of one of these, the Easton Terrane. The basalt and
its overlying coating of sea floor sediment were carried beneath North America and subjected to
pressures of around 7-9 kilobars, equivalent to 23-30 km (1 kb ~ 3.3 km) and temperatures of
330-400C (Brown, 1986). The rock was metamorphosed by the heat and pressure, became
accreted to the margin of North America, and then was rapidly uplifted to upper crustal levels.
During metamorphism, minerals were stretched out and aligned, so the rock may look layered in
places. This is metamorphic foliation, not sedimentary bedding. The green color is from the
metamorphic minerals, principally chlorite, actinolite, and epidote, that replaced the original
minerals in the lava. The rock was deformed, and in places was folded or stretched out. It was
never hot enough to melt. Wavy veins of quartz help accentuate the deformation of the
greenschist. The quartz probably grew in fractures during the earliest stages of metamorphism,
before the rock had been carried very deeply by subduction, and was still relatively cold and
brittle.
Hard to get any greener than this! These foliated slabs are just south of the bridge on the west
bank.
John Scurlock photographs the eroded tree stump in growth position beside the greenschist slabs.
While you are down at the river, see if you can find the huge eroded roots of a tree emerging
from the river gravel; it is right next to the best outcrops of greenschist. The stump is in a growth
position, indicating that it was not drifted here but is in situ. The remains of this tree are evidence
that the river channel has migrated across the valley floor.
The problem of Lake Creek’s sediment fan
John Scurlock at the Lake Creek foot gbridge. The stream has cut down into its own fan.
Stay on the Baker River trail- don’t turn right across the bridge. You’ll pass a few places where
recent debris flows and floods (perhaps in 2006) left a pavement of fist-sized stones on the forest
floor. Trees and heartier shrubs poke out of the rubble. In around ¾ mile, cross Lake Creek
(footlog when we were there) draining Shuksan Lake, perched in a hanging glacial cirque at
3694 feet. Notice that the creek has cut deeply into its own forested debris fan, leaving high
banks of alluvial sand and cobbles. Walk a bit further and find yourself in a clearing at the top of
the river bank, 20 feet or so above the gravel bars of the river. Look back downstream. Find a
vantage point where you can just see the mouth of Lake Creek. Notice that the river bank slopes
downward away from this tributary creek, especially evident downstream from the mouth of the
creek. If a creek deposits a fan, then erodes a channel into it, this indicates that the base level of
the creek has changed. Base level is the elevation at which a creek ceases to erode because it has
reached the level of its mouth. Ultimate base level is sea level, but here, while the creek was
depositing the alluvium several tens of feet above the modern Baker River level, the creek’s
outlet (intermediate base level) in the Baker River valley must have been at a higher elevation
than it is now. Something happened, and the creek’s entry into Baker River changed to a lower
elevation, so it ate downward into its own sediment deposit. What was the “something”? We are
at about 840 feet above sea level here, which may have particular bearing on this problem.
Lake Creek enters the Baker in the middle ground. The sloping surface of the fan/delta is
apparent, well above the river level.
Back at the end of the Pleistocene, the Baker River’s mouth down at the Skagit was blocked by a
huge fan of glacial outwash called the Burpee Hill Fan- you surely noticed the sediment within
this fan in roadcuts if you drove up the Burpee Hill Road from Concrete. This was sediment
washed off the thick lobe of the Vashon ice sheet that pushed up the Skagit valley from the
Salish lowland. The sediment spilled into the Baker valley, which was dammed by the ice to
form Glacial Lake Baker (Scott and Tucker 2006; Tucker and Scott, 2009). The ice receded,
leaving behind a remnant well into the Holocene, drowning the Baker River valley to an
elevation of— 850 feet! Ta da! So Lake Creek’s alluvial fan may have extended into this lake as
a delta, and the creek’s intermediate base level was 850’. After drainage of the lake, in the midHolocene, Lake Creek’s base level was lowered to that of the reformed river flowing across the
newly exposed lake floor, and the creek cut down into its own alluvial fan/delta.
Baker River
While you are at this river bank viewpoint, look at the Baker River. You are no doubt familiar
with sweeping, looping meanders in rivers, such as the flatter portions of the lower Skagit and
Nooksack Rivers as water slowly flows toward the ocean. At this vantage point there are several
channels, winding around, or cutting across, gravel bars. The bars consist of sediment carried
along the river’s floor in times of high water and high energy. This is a good example of a
braided river, which is most common where bedload (the coarser component of a stream’s
sediment load) is dominant. Look for logs stranded on these bars or along the river banks- these
mark the river’s surface levels during floods. The river is forced to flow around its own sediment
when water levels recede. The cobbles in the river come from a number of different rock units up
stream, plus a fairly high proportion of erratics from beyond the Baker River drainage, left
behind when the Baker River lobe of the various Pleistocene Cordilleran ice sheets melted away.
The unnamed waterfall.
Unnamed waterfall
The trail now leaves the river for good. Watch for a high waterfall cascading over the sheer rock
wall west of the river about 0.4 mile (600 m) beyond the river bank view point. It is especially
obvious if you are here when the trees are leafless. You can hike about 0.1 mile (150 m) cross
country up the alluvial fan to this unnamed waterfall. You may spot a faint foot path leaving the
trail on the left at or near 48° 46.398N and 121° 32.698 W. But if you don’t, then just push your
way up through sparse salmonberry brush and an elfin forest of moss-coated big leaf maples. It is
pretty easy going. You’ll gain 200 feet and be at the base foot of the fall on an eroded cone of
landslide rubble. This is a very worthwhile side trip.
Properly dressed geo-explorer for an early December visit to the Elfin Forest below the
Unnamed waterfall. (That’s John Scurlock.)
The water flows over Shuksan Greenschist. The visible part of the falls is a few hundred feet
high, but the topo map indicates it begins at around 3400 feet. There are a few channels eroded
by the unnamed stream. Even when we were there, in the wet season of early December, the
stream itself disappeared into its own alluvial fan.
Landslide rubble
This greenschist boulder lying on the alder tree shows that debris continues to come off the
valley walls.
Just beyond the landslide, the trail passes through a concentration of auto and even bus-sized
blocks of fern-draped greenschist. The near-vertical valley walls soar just beyond the pile of
blocks. The Baker River flows in a classic U-shaped glacial valley, with a flat valley floor filled
with coarse alluvium and a steep walls, scoured by the glacial rasp during repeated Pleistocene
glacial advances from the interior of British Columbia.
Sulphide Creek alluvium
Rocks at Sulphide Creek: two varieties of Easton Schist at top; volcanic breccia at right;
granodiorite at bottom. Greenschist is ubiquitous.
Pass the sign at the south boundary of North Cascades National Park, and walk the level trail
through soggy forest to the bank of Sulphide Creek. This creek drains the entire south flank of
Mount Shuksan, including the large Sulphide and Crystal Glaciers. This is the end of the trail.
Poke around among the rocks at the creek. Greenschist is the most prominent rock, but you’ll
also find granodiorite or tonalite eroded from the Chilliwack Batholith. There are a couple of
small granodiorite intrusions up Sulphide Creek (Tabor and others, 2003), but the contact of the
Shuksan Greenschist with the main body of this huge complex of intrusive rocks is another
couple of miles up the Baker River valley. You’ll also see some dark, swirled schist with folded
quartz veins. This is Easton Schist, the metamorphosed sea floor sediments that were subducted
along with the Shuksan Greenschist. Especially watch for sparse blocks of volcanic breccia: a
dark gray or black matrix containing angular fragments of lighter rocks. It is a good idea to break
one of these open with your ever-present rock hammer. The dark part of these rocks is
microscopic volcanic ash and crystals. The large clasts (rock fragments) are a mix of schist,
maybe some Chilliwack granitics, chunks of quartz, and very fine grained older volcanic rocks.
There is a small outcrop of rock like this near the top of Cloudcap Peak (map unit Thmb of
Tabor and others 2003), drained by Sulphide Creek; it is probably related to the Hannegan
caldera (Tucker, 2006; Tucker and others, 2004). The caldera itself is a great mass of breccia on
the other side of Cloudcap Peak, just outside the Sulphide Creek drainage area. The Hannegan
rocks fill a 3.72 million-year-old collapse caldera, similar to the younger Kulshan caldera
between Mount Baker and Mount Shuksan (Hildreth, 1996; Hildreth and others, 2003). The
breccia formed as gigantic ash eruptions tore upward through bedrock, carrying fragments of the
older rocks with it. The mix of ash and rock fragments filled the subsiding caldera structure:
Hannegan caldera contains 1000 m of fill, some of it similar to these volcanic breccia blocks on
the bank of Sulphide Creek.
References:
Brown, E.H., 1986, Geology of the Shuksan Suite, North Cascades, Washington, U.S.A.:
Geological Society of America Memoir 164, p. 146-154.
Scott, K. and Tucker, D., 2006, Eruptive Chronology of Mount Baker Revealed by Lacustrine
Facies of Glacial Lake Baker: GSA Abstracts with Programs, v. 38, no. 5
Tabor, R.W., Haugerud, R.A., Hildreth, W., and Brown, E.H., 2003, Geologic Map of the Mount
Baker 30- x 60-Minute Quadrangle, Washington. Geologic Investigation Series I-2660. US
Geologic Survey. Online: http://pubs.usgs.gov/imap/i2660/
Tucker, D., 2006, Geologic map of the Pliocene Hannegan caldera, North Cascades,
Washington: Geological Society of America Digital map and Chart Series 3 (accompanying
text), 3 p. http://www.geosociety.org/maps/2006-DMCH003/2006-DMCH003-TXT_E.pdf
Tucker, D. and Scott, K., 2009, Structures and facies associated with the flow of subaerial
basaltic lava into a deep freshwater lake: The Sulphur Creek lava flow, North Cascades,
Washington; Journal of Volcanology and Geothermal Research, v.185 p. 311–322.
Tucker, D.S., Hildreth, W., Ullrich, T., and Friedman, R., 2007, Geology and complex collapse
mechanisms of the Hannegan caldera, North Cascades, Washington, USA: Geological Society of
America Bulletin, v. 119, p. 329-342.