Bolton Peninsula II: Notes on the Twin River Formation

I posted an introduction to the Bolton Peninsula (bolton-peninsula-introduction) over a month ago and had not followed up with any further posts. This post is limited to the southwest portion of the peninsula and the very excellent exposure of the Twin River Formation.

Twin River Formation, Bolton Peninsula along with large glacial erratic

LiDAR image showing Twin River Formation exposure area

The Twin River Formation is named for its type locality approximately 50 miles to the west at Twin River on the north side of the Olympic Peninsula. The Twin River Formation is part of a series of sedimentary units that overlie the thick Crescent Formation basalts.

The exposure on the south end of the Bolton is outstanding considering how much of the formation is not particularly hard rock. The exposure is the result of the south end of the peninsula is eroded routinely by southwest wind generated waves moving up Hood Canal and eroding the base of the slope and transporting eroded sediment to the west and east sides of the Bolton Peninsula.

Birdseye (1976) http://www.dnr.wa.gov/Publications/ger_ofr76-26_geologic_map_jefferson_co_24k.pdf mapped this unit along the shores of the Bolton.

Close up of Birdeye (1976) map of site

Birdseye (1976) denoted the bedrock with a dashed line pattern. I have been all over the slopes in Section 31 of the area on the map above and found most, but not all, of the slope to the west of the road to be underlian by bedrock and found that the bedrock extends to the north nearly to the the north end of the secition line.

Twin River Formation silt/sandstone

Exposure of Twin River Formation on Bolton

As can be seen in the above images, the rock is fairly weak and readily crumbles. Not exactly outcrop forming material. Hence, the steep shoreline bluff slope on the southwest end of the Bolton provides an unusual exposure of the formation including readily measurable strike and dips.

Twin River Formation showing well defined bedding striking northwest and dipping to the northeast

Like many of the sedimentary units above the Crescent Basalts, the Twin River Formation contains concretions

Twin Rivers Formation on the bluff face and within the partially covered bedrock platform beach

Boulder of a block of Twin River Formation sandstone on upper beach

I did not find any fossils in the formation, but then I am terrible at spotting them and many may be at the microscopic level.