Geologic Hazards on Whidbey Island

As part of the Northwest Geological Society symposium on “Living with Earth Hazards,” we went to Whidbey Island on on Sunday, February 18, a cold, windy day, which started out snowy, then turned sunny.

We looked at evidence, most of it preserved and revealed in coastal bluffs, beaches, and marshes, of:

1. more than one glaciation of the Puget Sound lowland

2. a glacial erratic boulder of Chuckanut sandstone with a palm leaf fossil (those were warmer times!) brought south at least 50 km by the most recent glacier

3. layers of interglacial sediment that had liquified and flowed (earthquake? weight and water pressure from the subsequent, overlying glacier?)

4. a landslide that took out some homes (but not quite any lives) and thrust a sea wall and beach outward and upward into Puget Sound

5. a lahar (mudflow) that flowed far from an erupting volcano

(this layer was thin and hard to recognize - would need to take some samples to the lab to look under a microscope for pumice, ash, and so on)

6. thin and sketchy tsunami deposits in layers of marsh muck

(better samples were found in previous studies of the marsh involving multiple boreholes to see how the tsunami deposits varied across the marsh, seaward-to-landward, and lab work on the sediment with a microscope to analyze the sand and silt both within and separated from its marsh-muck context and to identify marine microfossils that the tsunami washed in from offshore)

7. An active fault zone with a scarp cutting across the island - a scarp not easy to recognize looking at it on the ground, but fairly obvious when viewed in LiDAR

(as was demonstrated to us by several presenters at the symposium and on this field trip, LiDAR is now considered an essential and necessary tool for finding and assessing active faults, and landslides as well - here's the Wikipedia article on LiDAR, but the section on how it is used in geology neglects to mention the importance of LiDAR in finding and assessing landslides)

(also, Brian Sherrod and colleagues dug a trench across this fault and studied it stratigraphically and structurally to about 10 feet deep - and then filled the trench back in and reclaimed it to the point that we could not spot where it was - "leave no trace," as Brian said)

The field trip was fun, but at the same time, it was sobering to see and think about the record of geologic hazards on various scales and frequencies that the Puget Sound area experiences, whether we want to or not.