Very large, glacially scoured Chuckanut slab, Samish Hill, Bellingham

By Dave Tucker  March 18, 2010

This site is destroyed by construction. See later post, here. Still worth reading about, at least!

The Samish Hills slab, looking across the entire extent to Bob Mooers at the far end. Click any photo to enlarge.

A remarkable slab of striated Chuckanut Formation sandstone and a little pebble conglomerate lies in a construction zone on the crest of Samish Hill. Deforestation and land clearing began in 2009; the developer completely stripped the soil and glacial overburden to expose a surprisingly large, nearly flat, glacially-smoothed rock surface. The site is being developed as the future home of Beth Israel Synagogue. There are a number of remarkable geological features now exposed to human eyes for the first time; this place is one of the geological treats in the area, and well worth a visit. If you are a Geology or Earth Science instructor, this would be a fabulous field trip. Thanks to my friend and neighbor Bob Mooers of Bellingham for taking note of this place and taking me there on a beautiful early spring day.

How to find the 'Samish Slab'. Click to enlarge

Getting there: The ‘Samish Slab’ is at the dead end of San Juan Boulevard, and near a nexus of trails from various neighborhood streets. To get there by vehicle or road bike, go west on San Juan Blvd off Yew Street Rd. Go to the end of the street, and walk south along the gravel construction road and a foot path about 150 yards. Or, get to the site via a number of foot trails through open space or undeveloped areas in the Samish Hill neighborhood; see the map. From the Samish Way overpass of I-5, walk up Elwood a block to 40th, zig quick left than right, and pick up a trail from the end of the street (Dumas). Walk through the woods on well-used foot trails. From Lakeway, go up Puget to Consolidation, and walk trails from the end of 47th or Racine. The nexus of trails shown on the map SW of the marked slab is within site of the clearing.

Geologic map from Lapen, 2000. Green square marks area of the Samish Slab. Note syncline fold mapped with red line and arrows indicating converging dip of the sedimentary strata.

Walk into the cleared area, either from one of the trails on the periphery or from the end of San Juan Blvd. Note that this is private property, though signs are few; I noticed many neighborhood kids and dog walkers blithely strolling through the clearing, heedless of the criminal act of trespass. The slab sits near the southeast corner of an area that was (re)logged and leveled in 2009. The developer has stripped 3-5 feet of overburden to reveal the glacially striated slab. This will be site of the new building. We should be glad this was done, because it has revealed this wonderful geo-treasure. Bob paced off the dimensions, and reports that the exposed slab is about 200 X 135 feet in extent, a remarkably continuous nearly flat exposure. The bedding appears to be essentially horizontal here, dipping only very slightly to the west, consistent with geologic mapping that shows this is along strike of the floor of a gentle syncline (Lapen, 2000). The bulk of the slab is within a single sandstone layer.

 

Bob Mooers examines at remarkably parallel and close-set glacial striae on the surface of the slab, bearing just west of south.

One of the first things you notice is the many shallow grooves and scrapes. One very numerous set is closely spaced parallel 1-3 cm deep grooves, oriented N30°E and S30°W, at a right angle to the strike of the bedding. These are mostly filled with a thin veneer of fine-grained dust, washed across the rock slab’s surface. I believe that these are glacial striae, made as cobbles and boulders were dragged over the soft rock surface during the last glacial advance from the north. There are also many more randomly oriented scrapes and scours, with sharp right-angled margins and sometimes with close-set parallel interior grooves- these are from ‘dozer blades stripping off the overburden.

View NE of the central channel eroded into the slab of Chuckanut. Look on the northern channel slope to see bedding. Water flows toward the viewer.

Perhaps the most remarkable feature, other than the sheer size and smoothness of the exposed slab, is the 3-4 foot deep eroded drainage channel running N40°W, right across the center of the slab. The northern margin of this channel is a gentle steepening of the slab as it dips down into the channel. Bedding is visible on this surface, and reveals a few  layers of medium-grained sandstone and cut-and-fill beds of cross-bedded pebbly conglomerate.  The southern margin of the channel is a 1-foot-high, steep, sharp rim, but not very straight. There is no fracture or fault or bedding plane exposed on the floor of this smooth channel. The channel is parallel with a host of narrow cracks, which are joints striking generally N40°W, parallel with the strike of the bedding and the fold axis of the syncline. These developed to accommodate stress in the brittle rock as it was subjected to the strain of regional folding during accretion of the Crescent Terrane (Olympic Peninsula).

Joints fracture the slab. These are parallel to the strike of the bedding (not visible here). Shallow glacial striae run across the joints.

How did this channel form? The floor is as polished as the rest of the entire slab, either eroded by the ice or by later water, or more likely, both. I forgot to look for striae in the shallow eroded channel. There may be a joint crack running along the base of the southern ‘wall’, but it is filled with fresh sediment; most of the floor of the channel is free of sediment and is smooth and unfractured. There was a slight trickle of water in the channel when I visited, draining the frog-rich swamp just to the east of the clearing. What a croak-aphony! The bedrock floor of the channel may have been at least partially exposed if the swamp drained this way prior to development. The developer has cleared out this channel eroded in the bedrock to help the cleared area drain. A drainage ditch has been dug from the bottom end of the channel and leads to a deep, artificial wetlands mitigation pond excavated into the overburden. When I visited, this 15-foot-deep pit did not have any bedrock visible in its bottom, and little water.

 

Deformed bedded chert in an erratic NE of the slab.

The other highlight of this site is the heaped erratics, salvaged during leveling of the site (a neighbor told me it used to be ‘hilly’). These are piled around the periphery of the clearing in heaps. Most of the glacially-transported boulders are pretty run-of-the-mill granodiorite. However, there are a few more interesting boulders, and a couple of oddball gems. In one heap northeast of the excavated slab is a nice hunk of our old friend Jackass Mountain conglomerate (see the Donovan Avenue erratic story). If you find that one, look in the same pile for a smaller boulder of ribbon chert (photo). I haven’t seen that before as an erratic. On the other side of the big clearing, by the future pond, is a pile with several nice boulders of pumiceous tuff (photo). I don’t know where these came from, other than obviously not around here. I’ll take a wild stab and suggest that a candidate could be the early Miocene Coquihalla caldera north and east of Hope, BC (Berman and Mathews, 1980). There are other tuffs in south central British Columbia, so this is admittedly a shot in the dark. I hope these really rare erratics are preserved as landscaping at the site so geologists can continue to see them. They are really different.

Detail of a tuff breccia erratic near the pond excavation. Pencil points at pumice clast.

Visit this site, and soon, before summer when work resumes. According to the site engineer, the slab will be excavated below current grade to serve as the foundation of the new building, so the smooth slab is doomed. This is the largest, smoothest, flattest Chuckanut I have ever seen, and probably the best place in Bellingham to see massed ranks of glacial striae. The slab provides a snapshot of what lies closely beneath the surface of  virtually all of Bellingham, indeed the entire western lowlands of Whatcom County, once you get past the glacial and post-glacial sediment overburden. Imagine the last, mile-thick glacier, grinding across the surface, in many places scouring away the rubble left by its several predecessors, right down to bedrock. That bedrock was never exposed to daylight until now however, as it was buried by till and outwash as the ice stagnated and melted. Go see this while you can; construction will completely destroy this site soon. Easy come, easy go.

Reference:

Berman, R.G., and Armstrong, R.L., 1980, Geology of the Coquihalla Volcanic Complex, southwestern British Columbia: Canadian Journal of Earth Science, v. 17, p. 985-995.

Lapen, T.J., 2000, Geology of the Bellingham 1:100,000 quadrangle, Washington: Washington State Department of Geology and Earth Resources, Open File 2000-5; maps and text.

4 Responses

  1. Dave, thanks for posting this, and thanks to Bob for “discovering” it. What a shame that it can’t be preserved “as is” for a geological site, and after all it is on privately-owned property. But at least the new building, built directly on bedrock, will have a larger degree of seismic resistance that most buildings in our region (built on sediment).

  2. Love the hikes but hate printing out all that paper to take with me when I explore. Can you please provide a way to print your data more efficiently? Thanks!

    • My idea to reduce paper is to copy the text and images only from the field guide websites [not the whole webpage with all the tabs and other hoo haw] and paste them into a word document [or equivalent]. Before printing, reduce font size, maybe reduce the photo sizes, and print on two sides. Any other suggestions out there?
      dt

  3. Dave (and “bookbikehike): I have done as you stated to “ompact” the text, Dave, and it works perfectly. By copying just the text I want, I can past to a Word document to have the field trip and not use too much paper. This is especially important, because as a teacher, I make copies to give to my students. They have it really easy, because traditionally for field trips I pose questions and I have them ” figure it all out,” but Dave, you do it for them in your excellent writing here. But nevertheless, I ask them to give written open-ended comments about what they learned when they take these trips, and they learn lots! Thanks so much for what you are doing with this blog (sorry for the rambling just now).

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