Glacial Geology of the northern Olympic Peninsula

Long, William

The highest stand of the Cordilleran Ice Sheet in the northern Olympic Mountains during the Vashon glacial maximum is recorded by the highest position of erratic stones derived from western Canda. Granite stones from Canada occur up to 3,550 feet on the north ridge of Mount Zion, which was the place on the northeastern peninsula where the ice sheet bifurcated into two lobes, the Puget Lobe floating southward into the Puget Lowland and the Juan de Fuca Lobe flowing westward to the Pacific Ocean. The margin of the Juan de Fuca Lobe on Blue Mountain, 12 miles to the west, stood 300 feet higher than on Mount Zion. The ice margin on Snider Peak, 40 miles farther west, was 1,100 feet lower than on Blue Mountain, indicating that an ice divide lay over the coastal lowland near Port Angeles. The Juan de Fuca Lobe advanced 11 miles into the Dungeness valley, 5 miles into the Gray Wolf valley, and terminated in a deep glacial lake ponded in the Elwha valley. At glacial maximum the Juan de Fuca Lobe reached within a mile of Forks in southwestern Clallam County. The Juan de Fuca Lobe blocked the former east-drainage of Lake Crescent and entered the lake basin but did not occupy all of it. The ice blockade created glacial Lake Crescent which expanded westward and filled the upper Soleduck Valley which, in turn, was blocked by continental ice advancing eastward up the Soleduck Valley. While the ice sheet occupied the Soleduck Valley at least as far east as Bigler Mountain, the outlet to glacial Lake Crescent was over terminal ice in the Kugel-Bonidu embayment and down the North Fork of the Calawah River. The North Fork Calawah proglacial drainage channel carried much of the meltwater from the ice sheet on the northwestern peninsula. Vlalley glaciers had retreated from their maximum positions prior to the arrival of the Juan de Fuca Lobe during the Vashon Stade. While the Soleduck glacier was retreating and building outwash in the upper Soleduck Valley, the Juan de Fuca Lobe was advancing east up the Soleduck Valley. The adjacent continental and alpine glaciers discharged outwash sediments into glacial Lake Crescent, resulting in sediments containing both foreign and local rock types. The upper Soleduck Valley outwash sediments were truncated by west-flowing glacial streams from the Lake Crescent basin following partial retreat of the Juan de Fuca Lobe. Truncation occurred when the terminus retreated and opened the Kugel-Bonidu embayment but still blocked the Soleduck Valley and the east-drainage of Lake Crescent. Evidence for a Vashon advance of alpine glaciers is indicated by alpine till overlying Vashon proglacial lake sediments in the upper Dungeness River valley. Gray Wolf and Dungeness valleys have glaciated U shapes in their upstream parts, but their downstream parts are stream valleys with V shapes, suggesting that valley glaciers never reached the coastal lowland. After the time of the Vashon maximum, alpine glaciers twice readvanced and deposited moraines well behind the mountain front.