The eruption of Mount St. Helens resulted in a gray, dense, pasty layer, 3–4 cm thick, on the bottom of Moses Lake, Washington. The ashlayer was easily recognizable in sediment cores either visually or chemically by its low calcium and water content. The layer fractured and sunk through the existing uncompacted sediment; apparent sinking rates were greater at 5 m than at 10 m depth. Sinking may not be the explanation for displacement at the shallowest (3 m) station. Although variable, the ashlayer had stabilized at the 10 m and 3 m stations after two years at average sediment depths of 5.6±1.1 and 7.9±4.4 cm, respectively. The relatively rapid sinking of the ashlayer through the sediment column at the 10 m and 5 m depths was probably due to its greater density (60–70% solid) than normal lake sediment (20–30% solid). Displacement at the 3 m depth may have resulted from a combination of greater rates of ash input and resuspension. Use of an ashlayer as an accurate marker for sedimentation rate calculations must include a consideration of such displacement.