The Biggest Rock in the Smallest Park

In a tiny town in king county Washington, life is quiet. Duvall does have one relic of past catastrophe, King County’s smallest park, ironically named Big Rock (Babcock and Carson, 2009). However, the past that brought Big Rock to its quiet resting place is infinitely fascinating. Big Rock is an erratic, likely dropped in a glacial valley of the cascade foothills carved out during the Fraser glaciation thousands of years ago, and the Big Rock is not all the glacier left behind, the valley is filled with mementoes of our last glacial period (Waitt, 2017; WDNR).

Duvall lies in a glacial valley of the foothills, with a river snaking down the center. The valley was previously thought to be carved by flooding that dropped big rock, but upon closer examination two telltale signs disprove that. First, the shape of the valley. The valley is U shaped, where as flood carved passages are typically not as rounded. Second, if you look at other glacial erratics they are deposited in a suspiciously linear fashion, specifically a moraine (Waitt 2017).

Big Rock is a metamorphic granitic rock different from all surrounding material, with scratch marks inconsistent with river transport. Therefore it must have been plucked, tumbled and subsequently dropped by a glacier, and upon recession the glacial melt began to form the river.

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