Tubificid oligochaetes selectively ingest silt- to clay-sized particles at depth within the substratum, transport them vertically upward through their gut, and deposit them as feces at the sediment-water interface. These activities form three distinct sedimentary layers. The sediment-water interface becomes covered with sand-sized fecal pellets. A silt-clay layer forms directly below this. The third layer is a sandy concentrate that represents the zone of tubificid feeding. The upper, pelletized layer is enriched in water content and organic carbon. The high water content of this layer, its irregular surface, and the low density of the constituent pellets destabilize the sediment surface and increase its erodability. In addition, the coarse-to-fine layered structure of the deposits forms distinctive biogenic graded bedding that is a potentially useful indicator of low current velocities and low rates of inorganic sediment accumulation in ancient fluvial environments.