A recent shallow landslide on a steep Whidey Island bluff had stripped off all the vegetation and top soil from the slope providing a good exposure of the underlying geology units.
Most of the bluff at this slide site is underlain by non glacial sediments that were deposited prior to the
last glacial period. The non glacial sediments consist of alluvial sediments with
layers of silt, sand and gravel as well as peat - the peat being a strong indicator of a non glacial environment. The non glacial sediments are
likely Olympia-age (60,000 years to 20,000 years). The sediments are very
compact and hard due to having been compressed by approximately 4,000 feet of
ice during the last glacial period. As such the sediments can hold a very steep
slope for long periods of time. The hard freeze and then rapid snow melt in February played a role in being final trigger for this slide.
Near the base of the slope I observed a sand and gravel layer. Even though this unit has been compacted by the over riding glacial ice as well as an unknown thickness of sediment before the ice arrival, the unit lacks silt and clay and thus I was able to easily collect some sand and gravel.
The most dominant pebbles in the gravel were irregular lumps of quartzite.
The conglomerate in the base of the Chuckanut Formation some 20 miles northeast of this site is full of similar pebbles. The quartzite pebbles in the base of the Chuckanut are derived from weathered Darrington Phyllite. The rivers that deposited the Chuckanut eroded weathered phyllite and concentrated the hard resistant quatzite into gravel deposits which became the basal conglomerate of the Chuckanut Formation approximately 54 million years ago.
The pebbles in the Olympia-age sediments can be viewed as geologic history repeating itself. A river system eroded the Darrington Phyllite and carried these hard resistant pebbles westward. There would have been no Skagit Bay between what is now Whidbey Island and the Skagit River. The Skagit River likely extended well to the west before the last glacial period removed much of the large alluvial plain that would have extended out from the Cascades to the west.
2 comments:
Is there a place where I can see the Chuckanut basal conglomerate?
The best spot I have seen is in a drainage above Cain and Reed Lake, but most areas in the lower sections of the Chuckanut over the metamorphic rocks of the Northwest Cascades will be rick in quartz pebbles. The same is true of the Huntington Foramation on Sumas Mountain. Quartz pebbles in the lower units in the Nanaimo Group may also look similar but also have a more continental signature.
Post a Comment