EPHRATA ERRATICS FAN

Ice Age Floods National Geologic Trail

The Ephrata Erratics Fan is a depositional area south of where water from the Missoula floods poured out of the lower Grand Coulee.  It is called a fan because the deposit is spread out like a fan or delta.

The basalt and granite boulders now littering the Ephrata Fan were carried there by torrents of water that gushed out of the Grand Coulee. Water and debris exploded from the mouth of the Lower Grand Coulee complex sending debris in a wide swath like pellets from the mouth of a shotgun. Velocity reduction at the coulee mouth and debris momentum carried large boulders a mile or more before they began to settle out of the slowing water stream. The largest of these, “Monster Rock”, is estimated to be about 8m (25 feet) in diameter and contains over 500 cubic yards of rock that weighs over 1,500 tons!

“The Ephrata fan is an immense accumulation of gravel and sand that resulted when mega-flood waters from Crab Creek, Dry Coulee, the lower Grand Coulee (ending at Soap Lake), and smaller scabland channels entered the Quincy Basin. The deposit probably formed more in the manner of an immense expansion bar (Baker, 1973a), rather than a fluvial fan in which relatively small alluvial channels shift across the fan surface without ever inundating the entire surface at once. Local areas of surface scour occurred on the fan, the most prominent of which is Rocky Ford Creek. The scour probably developed during waning flood stages, when draining of the inundated Quincy Basin caused relatively steep water-surface gradients to occur over the depositional surfaces that had been constructional during the high stages of megaflooding (Baker, 1973a). The scour processes produced the lag concentration of boulders on the fan surface, many of which can be seen from this viewpoint. An alternative explanation for the morphology of the Ephrata fan is that it was progressively incised by a sequence of multiple floods of successively decreasing magnitudes (Waitt, 1994; Waitt et al., 2009). It may also be that a more complex combination of these mechanisms occurred.”

Due to sudden expansion, the floodwaters decelerated and deposited about 130 feet of sediment onto the fan. At the head of the bar, east of Ephrata, are house-size boulders up to 60 feet in diameter. Sediment sizes in the fan decrease with distance south from the mouth of the Grand Coulee.

Many of the large boulders that cover the fan are basalt that was likely ripped out of Grand Coulee and other scabland channels just upstream. There are also numerous granitic erratics that were likely carried in the floods from as far away as the Grand Coulee Dam area, over 6o miles north.