Wednesday, July 17, 2013

Kinney Point

Kinney Point is the southern tip of Marrowstone Island and is actually a Washington State Park. It has no upland access, but it does host a Washington Water Trails campsite.  On my long walk around the south end of the island, this was the final turn before the home stretch back to the car.

Kinney Point is a double point, with a little notch in between.  As with so much of this end of the island, most of the upper bluff is glacial drift, but the bottom few feet is older sedimentary rock. I think the shape of the point reflects a change in the resistance of the toe to wave action.  A layer of sandstone anchors the eastern bump of the point, but it dips to the west and vanishes below beach level, exposing what appears to be a more erodible, finer grained unit (mudstone?) on the western bump.  At the same time, the amount of water seeping out over this unit increases significantly in the same direction and maybe that also affects erosion patterns.

Google Maps:  AERIAL VIEW
Ecology Coastal Atlas:  2006 AERIAL PHOTO

On the western side of the point, both the till and the older rocks seem pretty messed up, and my gut told me they were suggesting a more complicated story than the previous several miles of fairly predictable layer cake geology.

There are plenty of things that influence the shape of the coastline.  Around here, the primary factor is the basic form it inherited from the glacier.  But wave action, the resistance of the shoreline to erosion, and the width/height of the beach (which buffers the effect of wave action), are all important in controlling relative erosion rates and therefore the evolution of the shoreline. Here on the south end of Marrowstone, the overall shape reflects its glacial origins, but it seems like the geology of the bluff toe (largely influenced by the presence and type of harder pre-glacial sediments) is an important factor in some of the secondary bumps and wiggles.

Heading back northwest, the beach changes dramatically.  The bluff vanishes entirely at the campsite and there is even peat and wood exposed on the beach, probably the abandoned relic of the marshy swale still seen at the modern shoreline. Continuing north, the beach grows to become a barrier beach that extends 500 meters along the shoreline before reconnecting with the bluffs downdrift.

Google Maps:  AERIAL VIEW
Ecology Coastal Atlas:  2006 AERIAL PHOTO


Dan McShane said...

Thanks for exploring this area and spending the time writing it up. I have been on the southest shore but had not made it entirely around the south end of the islnad.
Based on my own observations, the toe of the slope south of the Lip Lip is note bedrock but is very hard and compact pre glacial sediment that is highly resistant to erosion.
Another erosion factor that I sometimes forget is wind. The upper sandy uits on the southern portion of the island are wind eroded and this in part explains the double bluff appearance. Not a common feature in western Washington but still a shoreline erosion factor. I measured a layer of sand 8 inches deep above top soil at the top of the bluff on the southeast shore of Marrowstone.

Gravel Beach said...

Thanks. I should check the geologic mapping more closely. Whatever marks the toe on the south end is awfully resistant stuff, and there are what appear to be both sandier and finer layers, but perhaps it is simply pre-glacial and not pre-Pleistocene!

Interesting about the wind erosion - I'm well aware of it on NW Whidbey and a few other places, but couldn't tell here.