Sunday, July 1, 2018

Possible Wave Cut Slope on Marrowstone

Much of the shoreline bluffs on Marrowstone Island are very abrupt, with a steep vertical drop due to the hard glacial till that can stand for decades as vertical cliffs. 

Stepping off the bluff edge on this bluff would hurt
Bluff is 65 feet and mostly silty glacial till
Many of the shoreline bluff I have inspected on Marrowstone have this sharp edged drop at the top edge of the bluff. The bluff above is exposed to fairly frequent wave energy such that any material that calves or ravels off the bluff face is washed away. That said, the bluff retreat at this particular bluff can not be discerned comparing historic aerial photographs dating  back 70 years. 

A recent bluff I was inspecting was even higher than the bluff shown above; however, it was not as steep on its upper part due to the higher amounts of cobbles and gravel within the upper bluff. About 20 feet down the slope it became vertical similar to the bluff above.

Test pits for septic systems often save me a bit of time for looking at the surface geology of sites.   

The above two pits were excavated into an area mapped as glacial till. At depth, the soil does become siltier and more typically of poorly sorted glacial drift observed elsewhere on the island bluffs as well as the steep cliff bluffs at Port Townsend. The round cobbles and slightly stratified nature of the upper units suggest some reworking of the sediments and forming a gravel/cobble lag. Glacial till can have some stratified layers due to water flowing at the base of the ice, and that was my working theory as to the high gravel and cobble  content in the test pits.

But on the drive over the mid section of the island, this slope is suggestive of a possible other explanation.

Maybe this higher portion of Marrowstone was reworked by wave action during seawater incursion during the late stages of the last glacial period.

The mass of glacial ice that filled the Puget lowlands caused the land surface to deform downward hundreds of feet. When the ice began to pull back, there was brief period when the sea invaded the lowlands and flooded the area at relatively higher levels than the current local sea level because the land surface was lower due to the ice loads. The water formed shorelines along the edges of areas inundated. After the land rebounded, theses former shorelines were uplifted to their current elevation.

These wave cut terraces are often rather subtle features. They are readily apparent in higher quality lidar (lidar-wave-cut-terraces-on-orcas-island).

Lidar imagery of southern San Juan Island
These wave cut terraces are pretty easy to see
The wave cut terraces on San Juan Island were recognized by geologists over 100 years ago (Bretz described them) as they are readily apparent. 

There is lidar coverage of Marrowstone; however, the quality is not as good as the more recent lidar coverage elsewhere and it is not clear if the slope above is a wave terrace.

But the slope is at the right elevation for having been a former shoreline. The hill slope is 170 feet. Across the water on Whidbey Island. Polenz, Schasse and Petersen (2006) mapping of the Freeland Quadrangle recognized former shoreline features at 125 to 130 feet at Double Bluff and 180 to 190 feet at Greenbank. Projecting across the water to Marrowstone the daisy covered slope shown above is midway between those two sites.

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