Geology of Squires Lake and the Alger Alp

By Dave Tucker

March 24, 2011

Map: USGS Lake Whatcom 7.5’ quad is very useful, although only the last bit of the trail is marked on it.  I have supplied a marked up version on Map 2.

Map 1. Courtesy Whatcom County Parks. Click to enlarge any image.

Squires Lake lies in a fold of the Chuckanut Formation less than a mile ESE of Lake Samish’s south end. It is the centerpiece of a joint Whatcom-Skagit County Park. For directions and overview of the park, go here (Whatcom County Parks), and see Map 1. Doug Madsen, a Bellingham physician, and an avid learner of geology, joined me at the trailhead for the 6-mile round trip to the summit of the bedrock knob that is ‘the Alger Alp’. The trailhead is just above the (unexposed, dang it) contact between the Jurassic or early Cretaceous metamorphic Darrington phyllite (Kdp on Map 2) of the Easton Suite, which underlies the sediment-filled Friday Creek valley, and the Eocene Chuckanut Formation (Ec on Map 2), rising steeply up the slope to the east. One of these rock units is metamorphic, and was once buried maybe 10 kilometers within the crust during subduction, and the other is a terrestrial sedimentary formation- that means that the contact between them is an unconformity, and I sure wish we could see it here. The contact is only known from a very few locations, none of them easy to get to; someday I’ll describe a trip to one of these, but I warn you, it won’t be easy to get to.

Squires Lake, view NW. It lies in a small syncline.

Hike the trail, which after a short climb joins the old F&S RR grade for several hundred yards. As you walk north, you are at the base of a near-vertical sandstone cliff of Chuckanut. You’ll eventually walk along the crest of this cliff. For now, you might be interested to know that the rocks dip away from you and the valley. Pay heed to the numerous big blocks littering the RR grade, fallen from the cliff above. These all obviously postdate the RR grade. At the lakes outlet, note that there is a concrete dam that is mostly submerged- the lake is natural, but it was ‘augmented’ in the 1920s, doubling its size. Walk the level trail around the west end and along the northeast shore. Take the ‘Squires Loop Trail’ to stay near the shore; there is an interpretive sign telling the history of the lake at a pair of benches on a rocky point between the lake and a swamp. The first interesting geological stop is a little beyond. The trail quickly rises several feet above the lake and becomes an obvious bedrock spine parallel to the shore, dropping steeply to the northeast. Look carefully at sandstone exposures in the trail tread and note the orientation of the beds. It is subtle, but worth the effort. These beds are tilted, and dip downward  to the southwest, toward the lake, at about 45 degrees. Keep this in mind as you continue the hike. ‘Dip’ is always ‘downward’, the steepest angle you can determine. It is the path a drop of water would take. Strike is the direction that a level horizontal line points when drawn on a dip face. See the sketch. In the photo, Doug Madsen demonstrates ‘right hand rule’ for determining direction of strike- fingers of his right hand point down the direction of dip, and his right thumb would then point in the direction of strike. Strike is not always easy to see in these rocks, as smooth bed surfaces are few and far between; a little geo-intuition may be needed.

strike and dip of tilted rock beds

You can detour on a loop trail to the Beaver Pond, which has a beaver stick dam across the outlet. The geomorph at the rather large ‘pond’ is interesting, because the 7.5 USGS topo (Lake Whatcom) shows the site of the pond as a waterless closed depression.

Doug Madsen shows how rocks in the trail dip toward the lake.

Hike around the far end of Squires Lake, on an old road bed. Take the South Ridge Trail, which climbs up an erosion channel. Just before the switchback, a 6’ cliff rises a few feet off the trail on the left. Walk over to it- which way do the beds dip? Right, downward toward the northeast, about 45 degrees. So, the two rock observations we have made so far indicate that rocks on either side of the lake dip toward each other, and

Map 2. Basic geology for Squires/Alger Alp by DT. Enlarge to see strike and dip symbols.

Doug Madsen and the NE-dipping rock layers near the South Ridge Trail switchback. View to northwest..

the lake lies in a minor structural trough, or syncline. The topo map shows that the swale the lake sits in continues across the ridge eastward, beyond Palmer Lake to Silver Creek; the little syncline is evidently an erosional low point. The rocks west of the lake dip downward to the northeast, as does the hidden contact with the phyllite at the foot of the hill along Old Highway 99. See the geological structure symbols I drew in on Map  2.

Branch hard left on the Pacific Northwest Trail (PNT), marked by white paint blazes. Follow these for most of the rest of the way. The PNT tread is on the very crest of the steep rock wall rising straight out of Friday Creek valley. There are several viewpoints. The quarry in the valley just below is the old Bell-Alger phyllite quarry, now belonging to a circus collective. You can see their big yurt. I-5 runs noisily at the foot of forested Blanchard Mountain, which is the western skyline. That whole mountain is Easton phyllite, semischist, and a few intruded igneous masses. You can learn about its geology by hiking the Oyster Dome field trip, also the unique  stilpnomelane and chert deposit trip.

Doug Madsen good naturedly provides scale beside the Road 1300 shale-sandstone interbeds.

The trail eventually becomes a logging road, marked ‘1340’ on Map 2, and then ‘1300’. Watch for road numbers on signs at junctions. The route curves past some blocky sandstone exposures, but generally runs on or near the crest of a bedrock ridge running NW- SE. This ridge is structurally controlled by a resistant sandstone bed in the folded Chuckanut rocks, which continue to dip to the NE about 45 degrees. Pay attention to the white blazes, and avoid spurs to left or right—stay on 1300 if you see signs for it. Note that the road surface is rounded pebbles- this is gravel hauled in to make the road; you are not walking on glacial outwash or till that is in place. I have seen only scant evidence of till along these ridges. The Chuckanut rocks weather to form thin sandy soil. Just beyond a spur numbered 1320, a nice exposure of interbedded shale and sandstone forms the low cliff on your left. The sandstone is pretty massive and little fractured, especially compared to the shale, which is very thinly bedded and weathered to little more than small rock chips. There are a few leaf fossils in the shale;  carefully to find anything intact, and then in the sandstone facies rather than the crumbly shale. Road 1300 curves a time or two and intersects Road 1000 at a T. The PNT and its blazes goes right and down the hill, but you go left.  You are now about 2.5 miles from the car, and near the end. The best rock exposure is just ahead, a 30-foot-high sequence of sandstone and shale beds, and includes some smudgy soft coal. The macrofossil scene here is the same as the preceding exposure: poor or worse. But the dipping beds are cool—they dip 45 degrees to the NE, and strike 216 degrees (that is, to the NW, as before).

The ‘coal bed’ outcrop on Road 1000.

The road curves around toward the forested top of Alger Alp. In decent weather you’ll have a view or two to the east across the Silver Creek valley to Anderson Mountain, which is also Darrington phyllite. There is a nice view of Mount Baker, even. Sherman Crater was steaming away when Doug and I did this hike. There is another Chuckanut- phyllite contact down there to the east, and in fact, the dip of the Chuckanut does again change, though you won’t see it on these jungled slopes. There must be the axis of a synclinal fold right about the top of the ridge, though I haven’t found decent rock to take measurements on. I have measured the dip of Chuckanut layers a little west of Cain Lake, and they dip to the SW. A representative strike/dip symbol is stuck in to the top right corner of Figure 2. Why ‘must’? Because the Chuckanut at one time covered the rocks of Anderson to the east, just as it overlaid Blanchard to the west. And the last time we saw the orientation of the folds, back at the ‘coal seam’ exposure, the rocks were dipping NE, downward in the direction of Anderson Mountain. So, to climb upward and overly the older and much higher Anderson rocks, the Chuckanut must reach the axis of a syncline and change over to a SW-dipping attitude. There is no Chuckanut up there now, it is all eroded away.  Folding uplifted the whole rock package, including the phyllite, and exposed all the rocks to erosion, mainly glacial, in the Pleistocene. It is possible that N-S normal faults run in the valleys on either side of Alger Alp and offset Blanchard and Anderson upwards relative to Alger Alp, but none are recognized by geologists.

Blanchard Mt. from the Alp. Friday Creek and I-5 are in the wide glacial valley in the middle ground. Chuckanut Rocks at one time covered Blanchard, too.

The answer to the location of the syncline axis is: it runs right along the crest of the ridge we have hiked up, and can be seen on a downloadable geologic map).  Oddly enough the LOW point of a structural trough folded into the Chuckanut forms the HIGH point of the ridge separating Lake Whatcom (including the valley east of Alger Alp) and the valley of Lake Samish. Just one of those quirks of where the resistant rocks are. See the cross section, below.

Schematic E-W section through Squires. See how the syncline forms the topographic high?

On the way back, at the junction of PNT and South Ridge Trail, you can go left and complete a loop around Squires Lake. Doug and I took 3 hours for the roundtrip, including plenty of geo-dawdling.