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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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Point Whitehorn, Whatcom County

 

Looking south at the Point Whitehorn Reserve beach. The dark erratic bottom left (with the yellow rock hammer) looks like Jackass Mountain conglomerate. Rock from this formation gets around!

By Dave Tucker. December 27, 2009 with UPDATES from November 2020.

Point Whitehorn Marine Reserve is a Whatcom County Park along the bluffs north of the Cherry Point refineries, and south of Birch Bay. The geologic highlights are 1) the till forming the bluffs at the beach, and 2) the great variety of erratic boulders on the cobble beach. There are nice views west over Georgia Strait to the islands- go for the sunset- or the storm waves, but NOT at a high tide! There is very little beach then.

Getting there: From I-5 Exit 266, drive west on Grandview Road 8.5 miles.  Follow the road as it curves left and becomes Koehn Road. Continue 0.5 miles to a parking area on the left. There is a 3/4 mile accessible trail through lowland forest, including a sizable (by modern standards) grove of large spruce, to overlooks atop the bluffs, with nice polished dunite benches. The trail then switchbacks 75′ down to the cobble beach.

Walk either way along the beach from the end of the trail. The beach is covered in cobbles- all are rounded erratics brought south from British Columbia by continental glaciers. If you go south (to the left if you are not so good with compass directions) toward Cherry Point less than a half mile, you will come across a recent landslide onto the beach. Saturated loose sediment, sticky with clay, brought trees with it when it collapsed. Perhaps the collapse was helped along by undercutting by storm waves. I wasn’t there to see it- goldernit! I wonder, did it make a noise?16568 rsz

Going north from where the trail reaches the beach, the highlights are the large erratics, some submerged except at low tides, and exposures of Pleistocene lake sediments and glacial till.

Figure 7 erratics at Point Whitehorn 111256

This pair of erratics are found just north of the trail. On the left is a ‘salt-and-pepper’ speckled crystalline granodiorite rich in dark, crystal-poor enclaves- read about those below. The greenish stone on the right looks like metamorphosed seafloor basalt. These two rocks formed in completely different environments, Eroded from their ‘home’ outcrops in the mountains of British Columbia, they were brought here by continent-covering glaciers in the past 15,000 years.

Watch for a really cool large, flat granodiorite boulder just a little way to the north, with glacial grooves or striae. The boulder has good examples of dark, mafic enclaves. Mafic is a mnemonic term for magnesium + ferric, because these dark blobs (OK, the technical term is blebs- you can look it up!) are probably magnesium and iron-rich basalt or andesite, compared to the more silica-rich and light-colored granitic rock of the boulder. We use enclaves because these dark blebs are interpreted to result from the invasion of hotter magma upward into the magma reservoir that was cooling toward a granodiorite composition. Bits of the hotter magma broke off the main intruding mafic mass and dispersed upwards into the granodiorite. The mafic magma is denser than the host granodiorite, but also hotter. The enclave’s tendency to sink was off set by the temperature difference, and they rose to some equilibrium level within the granodiorite magma. They may have been carried in ‘trains’ in hotter upwellings into the granodiorite body, as well.

The bluffs for the first part of the walk north are composed of glacial till, or sediment deposited directly from glacial ice. If the till was deposited directly on land, then it is a ‘let-down’ deposit as the glacier ice melts. The ice contains lots of dispersed clay, pebbles, boulders and sand. During melting that sediment load is concentrated, accumulating under and on the wasting ice surface until the ice is finally gone; then all that debris is left lying on the glacially scoured surface that underlay the ice. However, there is also glaciomarine drift associated with ice sheet deposits on the shores of the Salish Sea. These rained out of floating ice onto the sea floor; to demonstrate such an origin for the deposit at Point Whitehorn, someone needs to find some marine shells preserved in growth position among all the till, because marine life did exist beneath the floating ice. I looked, but not too hard (I was enjoying the watery sunset).

A little further north, a gully festooned with fish net floats breaches the till bluff. Just beyond, look for some bedded silty sand at the base of the till, right above the beach- there is a very sharp contact between the till and these very different-looking sediments. The bedded layers are slightly deformed. These deposits were only barely exposed when I first wrote this

20201031_141101 rsz mark

Rythmically bedded silty layers at the base of the bluffs at Point Whitehorn in 2020

field trip in 2009, but thanks to continued wave cutting at the base of the bluff, in November 2020 they are much more evident, and can be seen for several hundred yards now. They may be buried by later slumping, so go soon because… well… geology happens you know.

20201031_141113 crop

Here’s a closeup. The contact with the overlying till at the top is sharp, not gradational. (A bit of the till has slumped down over the bedded sediment at upper left.) The rythmically bedded sediment consists of alternating layers of coarser and finer silt; each pair of beds is about 6″ (15 cm) thick.

What do you think these are?  ‘Rythmites’, as such deposits are known, are generally associated with deposition in quiet water- ponds and lakes. The symmetrical, ‘rhythmic’ layering reflects episodic deposition of slightly finer and slightly coarser sediment grains. This may be due to changes in sediment load of streams entering the lake between summer and winter, for instance; in this case the paired layers are called ‘varves’ and can be used to tell the passage of time. If these are indeed varves, then I counted 37 pairs in the large photo above, equal to 37 years of sedimentation.

So, what was a lake doing here just before the glacier advanced and left till atop the beds? It is known from throughout the Puget Lowland that the great mass of advancing ice depressed the crust into the putty-like mantle far below, and this depression was accentuated closer to the ice. It is also known from deposits elsewhere that a great pro-glacial lake, Glacial Lake Bretz, formed in the present Puget Lowland as rivers flowing into the lowlands were dammed by the advancing ice; the spillway for this great lake was south of Olympia. Glacial meltwater certainly added large volumes of water to Glacial Lake Bretz. As the ice continued to advance, it overrode the lake sediment, and in the process deformed it. However, an equally plausible accounting for these lake beds at Point Whitehorn is a smaller, local lake, formed by drainage blocked locally by the ice. Lake Bretz is associated with Puget Sound specifically, not this area well to the north.

20201031_144508 rsz mark

Lake deposits thicken as you walk north. These are about 3/4 mile north of the trail down to the beach.

These lake deposits thicken and become more pronounced further north along the beach. But there are also sandy stream deposits in the bluff. Keep an eye out for a few exposures of crossbeds.

20201031_143206 rsz mark

Cross beds in sandy stream deposits at Point Whitehorn. Earlier sands deposited on a stream bottom are eroded off and buried by similar sand by currents. Sandstone derived from crossbedded sandy sediment just like this is very common in the Chuckanut Formation.

Fifty million year old sediment just exactly like these that have been lithified and cemented into rock make up much of the Chuckanut Formation, exposed in the hills around Bellingham- and buried far below these Pleistocene sediments at Point Whitehorn. Here is an example from Silver Creek in the Chuckanut Mountains.10636-mark

Then there are all the rickety stairways built by residents atop the bluffs above to access the beach. Very few of them are usable. Also note the corrugated plastic drainpipes, used to carry runoff from gutters and french drains around the houses over the bluff, in an effort to keep rain percolation from saturating the bluff and causing collapse. IN the long run, futile I fear. These houses are on Keene Rd, at the end of the road you drove down just before turning into the Point Whitehorn parking lot. You can see these houses on internet images, and measure there distance from the top of the bluff- the nearest is only 20′ from the brink, the furthest is 50 feet. Nice view- for another few decades. Like I said, ‘geology happens’.

20201031_142213 rsz mark

If any visitors have pictures documenting further erosion around this big erratic, send them to me.

I really don’t mean to harp on erratics, but I know many of my dear readers love these rocks. And everyone will have to admit this is pretty cool. All those big erratics further out from the beach were once encased in till like this one. And those ‘way-out-there’ erratics also mark where the bluffs once met the sea. Some day this big bugger will be poking out of the waves; and those houses will be loooong gone.

 

4 Responses

  1. Bob and I checked out this geology walk today. We easily found the huge boulder with the glacial scars on it and all of the granodiorite with the xenoliths .
    We also found the silty layers on the lower cliff which were irregular and under the layers with the cobble size rocks in them. Could they be wind blown sand dunes from a period between glaciations? We are clearly missing two things that real geologists have. One is a college degree in geology, the other is the ability to drink lots of beer and be creative with theories. Adena Mooers

    • Adena,
      You and Bob have the most essential qualities for geologists: curiosity, a strong desire to ‘go see’, and the ability to ask questions until you are satisfied with the answer, whether you come up with the solution or someone helps you with it. You clearly ARE creative.

      As to beer drinking, I won’t say it is over-rated. But it isn’t too late to develop the capacity.

      Happy New Year.
      Dave

  2. Dave, I think the sequence we see at Pt. Whitehorn Marine Reserve might be Vashon till covering preglacial advance outwash (well-sorted silt). The latter is called Esperance Sand down near Seattle, where it is quite conspicuous. Above the till is glaciomarine drift, which isn’t too evident right there because of growth of all those messy plants, which hide the stratigraphy! As I recall, a better sequence is found a bit further north closer to Pt. Whitehorn itself. But we’ll have to go out again and look, maybe with our imaginations fueled by some IPA!

  3. […] on behalf of the Whatcom Land Trust, who manage this beautiful strip of beach, bluff, and forest. Read my post about Point Whitehorn from a few years […]

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