Dickerman Mountain trail geology guide

Few people hike the famously steep Dickerman Mountain trail for its geology. The whole point is to grind up the interminable switchbacks in the woods and patchy meadows to get on top and enjoy the view. But knowing something about the rocks you are hiking through can ease the hike. At first glance the trailside rocks aren’t overly interesting unless you look at a fair selection of them, up close, to see their texture. There are a lot of rocks along the stony trail, so many opportunities.

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Getting there: Go east from Granite Falls on the Mountain Loop Highway. Pass the Verlot Public Service Center and continue along the Mountain Loop Highway 16 miles, to beyond the Big Four Ice Caves trail. The Dickerman trailhead is well signed with a large parking lot on the north side of the road; there is an outhouse and picnic table. A Northwest Forest Pass is required, and can be purchased in Verlot. Round trip: 8.2 miles; elevation gain, 3950′. (Yep, you heard right, just shy of 4,000 vertical feet, an average grade of 18% or 975 feet per mile.)

 

Geology from Tabor and others, 2002. Geologic units associated with the Barlow Pass Volcanics: dark brown ‘Tbr’= rhyolite flows and ash-flow tuffs; light brown ‘Tbv’= volcanic rocks, undifferentiated, but largely basalt lava; green ‘Tbsv’ = sandstone and volcanic rocks. Mountain Loop Highway in red; Dickerman Trail is also shown in red snaking up the steep mountain. Dip symbols indicate that the the entire volcanic unit tilts largely to the east 30-50 degrees. Dikes intruding the volcanics are not shown at this scale. Dark bar at lower left is 1 mile.

Geologic overview: The craggy bulk of Dickerman Mountain, and the surrounding area between the South Fork Stilliguamish River and the Sauk River to the east, is underlain by the Barlow Pass Volcanics. These rocks erupted from unknown source vents around 45 million years ago. They consist of stacks of thick basalt and basaltic andesite lavas, with some dacite and even rhyolite ash flows. There are sandstone interbeds and dikes. For a full description of these volcanics see page 28-30 of the pamphlet accompanying Geologic Map of the Sauk River 30- by 60-Minute Quadrangle, Washington, by R.W. Tabor, D.B. Booth, J.A. Vance, and A.B. Ford (2002). The best ages were obtained by fission track analysis in zircon crystals separated from rhyolite samples; these range from 44-46 million years ago (late middle Eocene). A small portion of the Barlow Pass Volcanics (as presently defined) is found northwest of Darrington, and consists of silica-rich ash-flow tuffs (ignimbrite), erupted violently from a long-gone volcano. According to Tabor (1994) the Barlow Pass Volcanics appear to overlie the Straight Creek fault without much offset, implying movement along this major regional fault had probably ceased. However, stretched cobbles in conglomerates of the Barlow Pass Volcanics suggest post-Straight Creek movement, perhaps during strike-slip movement along the Darrington-Devils Mountain Fault Zone.which runs down the South Fork valley. Today, glacially eroded exposures of the Barlow Pass Volcanics typically form high cliffs protruding out of the forest, and blocky summits with considerable relief. Peaks consisting of these volcanic rocks include Sheep Mountain east of Barlow Pass and noth of Monte Cristo, and Dickerman’s neighbors Twin Peaks, Stilliguamish Peak, and Mount Forgotten. Feldspar-rich sandstone found interbedded with the lavas and tuffs of the Barlow Pass Volcanics closely resemble sandstone of the Chuckanut Formation. Tabor and others (20o2) even suggest that future work may show that the volcanics should be included as a late-occuring eruptive facies in the Chuckanut.

Geologic guide:  The trail immediately crosses the toe of a large landslide, the blocks forested and moss-covered but still recognizable. Just beyond the landslide blocks, watch for rounded pebbles in a matrix of sand and silt in the trailcut. This is mapped as glacial till left behind as ice of the late, great Vashon glaciation melted away. I found a single quartzite pebble, carried all the way from British Columbia- quartzite is not known from the Cascades, and is an important indicator lithology to recognize deposits of the Vashon lobe of ice that originated in Canada.

White dikes cut basalt cliffs of Barlow Pass Volcanics on the south side of Dickerman Mountain. View from Big Four Ice Caves parking lot. Click to enlarge any photo.

The trail quickly climbs up through the forest. Within a dozen switchbacks you come face-to-face with a wall of basalt (?), a distinct rib of rock running up the slope for several hundred feet. Why this feature stands out so proudly I don’t know, though it could be held up at its core by a dike. It would take some scrambling around to try to find better evidence other than topography for such a resistant feature. There are whitish dikes cutting up through the basalt lava cliffs west of here, visible as whitish bands from the Big Four Ice Caves parking lot.

 

 

All of these rocks have secondary minerals filling the original vesicles. So, the texture is ‘amygdaloidal’, not ‘vesicular’. Notice variable colors- not helpful in rock identification.

There is no stratigraphy readily apparent on the mountain side.  The trail climbs relentlessly, with few views. But you are not distracted from examining the stones underfoot. Geologic curiosity provides a handy excuse to take a breather: “You go on ahead honey. I, ah, (huff, puff) really need to look at this fascinating rock.” Rock fragments (‘clasts’) comprise the colluvium that mantles the slope. These clasts are weathered and eroded from rock below the soil cover, or fallen from cliffs above. The colluvial cover is constantly creeping in very slow motion down the hillside- ubiquitous ‘pistol butt‘ tree trunks tell us that. Texture of these otherwise unimposing clasts indicates that they are volcanic. Many clasts contain quarter-inch or smaller vesicles that were gas pockets trapped within the lava; they are now almost always filled with soft, easily scraped white secondary minerals (probably calcite and zeolite minerals) that grew when water filled the gas cavities; the filled vesicles are called ‘amygdules‘. Most all the rock you will see is an uninspiring dark gray; if vesicles aren’t present, the rock is mostly homogenous and very fine grained. It takes a good handlens and experience to make out much of anything inside a clast. Beware of surface color of these rocks to help you learn more about the rock, as these rocks have been subjected to thousands or millions of years of weathering, which subtly alters the rock’s appearance. On top of that, the clasts are covered with a thin veneer of lichen or other vegetable that mottles the rock in greens, whites and grays.

 

Sandstone clast along Dickerman Trail. Click to enlarge. Dark minerals are biotite mica, light ones are plagioclase feldspar, and maybe a small amount of quartz.

Look sharply to find clasts eroded from sandstone interbeds- these are, well, sandy-looking when you look real close. The ones I noticed were salt-and-pepper sub-millimeter mineral grains in pale greyish-tan angular clasts. The well-cemented grains were probably eroded from ash flow tuffs after eruptions.

 

The swale marks a sackung fracture in the hillside. People at far end for scale.

The trees finally thin out and you traverse  northwestward below high cliffs. The trail crosses a stony-bedded drainage at around 4400′. Just beyond the trail runs SW for a couple hundred feet along grass in a narrow, shallow swale- the opposite side of the swale forms a slightly higher mound parallel with the depression. This topography is typical of ‘sackung’, (‘sagging’) hillsides that are slowly creeping outward. The depression is the sediment-filled fracture at the top of the slipping hillside block. These features form on steep unconsolidated slopes. They are a reaction to gravitational stress, pulling the slope surface apart. They may evenually lead to a landslide.

 

Two tephra layers. Knife is in the Mazama ‘Layer O’. Red layer below is older- maybe Glacier Peak, or Rainier. About 4460′ in meadows.

Just beyond, watch for a few exposures of young volcanic ash in black soil of trail cuts. I recognized one cm-thick, pale-colored layer consisting of slippery microscopic ash grains, an ash unit you may have seen in many other Cascade locations. I am pretty sure this is Mazama ‘Layer O’ ash from the great caldera-forming eruption at Crater lake around 7200 years ago. Just below is a sandier, reddish ash layer. This is clearly older than Layer O, and could be from Glacier Peak, though Rainier is a possible culprit, too.

 

 

A couple dozen lava flows are stacked on the west face of Dickerman Mountain. Summit is capped with ash flow tuff.

At long last reach a northward viewpoint along a rocky rib at about 4900′. The vertical wall of Dickerman Mountain’s west-facing glacial cirque, now glacier-less, consists of a couple dozen layered basalt lava flows, piled up like pancakes. At long-last the 5720′ summit is reached, one of several rocky knobs. The rock here is mapped as a thin veneer of rhyoliic ash-flow tuff, blasted out over the Eocene landscape during an explosive eruption…somewhere. These pyroclastic flows can cover a lot of ground, so the vent could have been anywhere, even dozens of miles away. The brown rock is measled with small round black blobs. These are also vesicle-filling amygdules, but unlike the white minerals filling the basalt below, these are hard. They are likely silica-rich, derived from water leaching silica out of the rhyolite tuff and filling the vesicles. Don’t forget take your eyes off the rocks and enjoy the view. If the weather is good, you can see a hell of a lot of mountainous scenery. If not, well, back to the rocks. Then the descent.

Dark amygdules fill former gas pockets in ash flow tuff at the summit of Dickerman Mountain.

 

 

 

REFERENCES:

Tabor, R. W., 1994, Late Mesozoic and possible early Tertiary accretion in Western Washington State: the Helena-Haystack melange and the Darrington-Devils Mountain fault zone: Geological Society of America Bulletin, v. 106, p. 217-232. Abstract on line.

Tabor, R.W., Booth, D.B. and Ford, A.B., 2002, Geologic map of the Sauk River 30 minute by 60 minute quadrangle, U. S. Geological Survey, Miscellaneous Investigations, I-2592, with pamphlet, 65p. scale 1:100,000, http://geopubs.wr.usgs.gov/i-map/i2592/