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).
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.