Washington
Washington
The Ephrata Erratics Fan is one of the most remarkable depositional features of the Missoula Floods: a vast, fan-shaped spread of boulders, gravel, and sediment deposited where floodwaters burst from the mouth of the lower Grand Coulee and...
The Ephrata Erratics Fan is one of the most remarkable depositional features of the Missoula Floods: a vast, fan-shaped spread of boulders, gravel, and sediment deposited where floodwaters burst from the mouth of the lower Grand Coulee and spread across the Quincy Basin. The fan contains hundreds of glacial erratics -- boulders carried from distant mountain ranges by icebergs in the floodwaters -- scattered across the landscape like a giant's marbles. Some of these erratics weigh many tons and can be traced to specific rock formations hundreds of miles away in Montana and Idaho. The fan shape is characteristic of a high-energy flow suddenly spreading and losing velocity: as the floodwaters exited the confined Grand Coulee and hit the open basin, they dropped their enormous sediment load in a spreading pattern. The resulting landscape is a treasure trove for geologists, with boulder types, sizes, and distribution patterns that reveal the floods' velocity, depth, and ice content. The Ephrata Erratics Fan makes the flood's raw power tangible: these are boulders that were carried hundreds of miles in icebergs and dumped on this prairie when the ice melted.
The Ephrata Fan is the depositional apron stretching south of the mouth of the Lower Grand Coulee, east and south of the town of Ephrata, Washington. Most of the fan is private agricultural land, but the largest erratics, including Monster Rock, sit on or near roadside parcels accessible via signed county roads and short pull-offs. No formal visitor facility; year-round.
The Ephrata Fan formed when Missoula floodwaters exploding out of the narrow Lower Grand Coulee onto the broad Quincy Basin abruptly decelerated. The hydraulic expansion dropped roughly 130 ft of sand, gravel, and boulders across a fan-shaped lobe. Sediment grades downstream from house-size erratics at the fan head (60-ft-diameter boulders) to fine gravel toward the south. The fan is one of the cleanest examples of a megaflood expansion bar and contains some of the largest ice-rafted erratics in the channeled scabland system. Monster Rock, over 25 ft tall and 60 ft across, is the headline boulder.
Mapping and granulometric work in the 2000s and 2010s by Bjornstad and others remains the standard reference. No major reinterpretation since the Balbas et al. 2017 chronology. No updates found since 2017.
Within the Lower Grand Coulee chapter's territory. The chapter has organized public field trips to Monster Rock and the fan head; no formal interpretive panel infrastructure.
Year-round; early morning low-angle light is best for photographing the larger erratics. Monster Rock is the obvious stop; the wider fan rewards a slow drive through the agricultural roads east of Ephrata.
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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