Washington
Washington
Although the Seattle region was not hit by the Missoula Floods that carved the scablands to the east, the Puget Sound has its own equally dramatic ice age story. The Puget Lobe of the Cordilleran Ice Sheet -- a sibling of the lobes that...
Although the Seattle region was not hit by the Missoula Floods that carved the scablands to the east, the Puget Sound has its own equally dramatic ice age story. The Puget Lobe of the Cordilleran Ice Sheet -- a sibling of the lobes that dammed rivers and caused the Missoula Floods -- advanced all the way to what is now Olympia, burying the entire Puget Sound region under ice roughly 3,000 feet thick. This ice sculpted the landscape into the drumlins, kettle lakes, erratics, and glacial till that define the region's geography today. Lake Washington, Lake Sammamish, and Puget Sound itself were all carved or deepened by glacial scouring. The Puget Lobe was part of the same ice sheet whose Okanogan and Purcell Trench lobes caused the floods, making this area a different chapter of the same ice age saga. Glacial erratics scattered across Seattle neighborhoods, the ridgeline of Capitol Hill, and the island-dotted waters of the Sound are all evidence of the ice that once filled this basin. The Puget Sound region proves that even where the floods did not reach, the ice age left its mark on every hill and waterway.
This is an IAFI essay on iafi.org (URL ends /34902-2/), not a place. The essay points readers toward physical sites in the Puget Sound region, most notably Discovery Park in Seattle and Snoqualmie Falls, where the underlying glacial story can be seen.
The article's thesis is that, although the Missoula Floods did not affect the Puget Lowland, the area has its own glacial-flood-adjacent story driven by the Puget Lobe of the Cordilleran Ice Sheet. The Puget Lobe advanced and retreated across the Puget Lowland at least seven times during the Quaternary. Around 18,700 years ago, the advancing lobe dammed Cascade drainages and created Glacial Lake Snoqualmie, which forced the Snoqualmie River over the basalt that now produces Snoqualmie Falls. Discovery Park's coastal bluffs in Seattle expose a layer-cake of pre-glacial fluvial sediments, glacial-lake clays, till, and outwash, the most complete onshore record of an advance–retreat cycle in the Puget Lowland. The piece serves as IAFI's framing of why the Puget Lobe Chapter exists despite the floods themselves being a Columbia Plateau phenomenon.
J Harlen Bretz's PhD on Puget Sound glaciation remains a touchstone; modern updates include Booth et al.'s INQUA volume contribution and ongoing UW/USGS sediment-stratigraphy work at Discovery Park.
Direct IAFI product, Puget Lobe Chapter.
Not applicable; this is the essay, not a place. For the on-the-ground experience, visit Discovery Park (free, daily) and Snoqualmie Falls (free, daily, ADA-accessible viewing platform).
Flood-extent overlay shows the maximum reach of the Missoula Floods.
The site in its place along the flood path, with the maximum flood extent drawn over the modern map.
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