Point Whitehorn, Whatcom County

Looking south at the Point Whitehorn Reserve beach. The erratic in the foreground looks like Jackass Mountain conglomerate. Rock from this formation gets around!

By Dave Tucker. December 27, 2009

Point Whitehorn Marine Reserve is a new 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.

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; there is 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.

If the till was deposited directly on land, then it is a ‘let-down’ deposit from the ice as the glacier 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 glacial 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 existed beneath the floating ice. I looked, but not too hard (I was enjoying the watery sunset).

Pebble-rich till overlies some nicely bedded silt at the bottom.

Pebble-rich till overlies some nicely bedded silt at the bottom.

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. What do you think these are?  Remember that at least in the latter stages of the Vashon glaciation, the wasting ice sheet was floating  on ponded brackish water. Were these beds deposited on the seafloor beneath the glacier? If so, why no ‘drop stone’ pebbles from the base of the ice? Do the beds predate the glaciation, settling onto the seafloor just ahead of the arrival of floating ice, then covered by a steady drizzle of till? Granted, the beds have very limited exposure at Point Whitehorn, so it’s hard to draw any kind of sweeping generalizations. Post your ideas as a comment.


3 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!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: