Ebey's Landing-to-Fort Casey Self-Guided Tour of Whidbey Island Geology
This stretch of beach includes the most complex and interesting example of shoreline bluffs in Puget Sound. It illustrates geologic sediments and processes ranging in age from a quarter million years to this morning. The accompanying vertical cross-sections are simply maps, but ina vertical plane. As such, they show the sequence of deposition of the sediments making up Whidbey Island, their mode of deposition, and, thus, the prehistory of the area. Some points of interest:
- From Ebey's Landing, a look north = The bluffs in the distance are composed of sandy gravel. Slopes are vegetated because they are protected from erosion by a barrier bar. Note the undulating dunes stop the bluffs (now covered by mature spruce trees), deposited since local sea levels stabilized about 5000 years ago.
- Thrust "faults" here and to the south are probably due to glacial rather than tectonic forces. The faults cut the sand making up the lower bank but die out in the massive(non-layered) silt making up the mid bluff.
- The glacial drift near beach level here is probably the oldest geologic unit in the northern Puget Sound. Note the concrete-like "till" at beach level vs. the gray silt above. The till was smeared onto the preexisting land surface by the ice sheet, whereas the overlying silt was deposited as the ice began to float. These silts contain marine microfossils (foraminifera).
- Elephant Seal Rock is a landmark that vividly illustrates seasonal and long-term changes in beach profiles. Note that the upper part of this glacial erratic is smoothed by the abrasion of wave-born sand whereas the lower portion, usually buried in beach sand, is still rather rough. Note the low-angle thrust fault in the lower bank here, resembling a mini-subduction zone.
- The yellowish tan formation making up the lower bluff here is a loess (wind—deposited Silt). Note that it is quite massive (non-layered), unlike silts we will see later, which were deposited in water. The greenish gray layer at the base of the silt is a fossil soil(paleosol). Paleosols formed when the deposition of the silt was slow enough to allow weathering processes to function. In some places there are additional paleosols higher within the Silt.
- Here the surface upon which the silt was deposited was a lake. In places here you will find small white and very delicate snail (gastropod) shells. These are fresh water species confirming the lake bed here.
- This wooded area is a large landslide. It probably was triggered by the concentration of ground water along the top of the gray lakebed silts. Note how level that surface is.
- Here we can see the sand that was deposited by melt water from the last ice sheet. This sand characteristically forms an angle-of—repose slope (about 35 degrees).Overlying the sand is the concrete-like glacial till, deposited directly from the ice sheet.