Types of layer 1 deposits in ignimbrite sequences include ground-surge, ground-layer, and jetted deposits. Ground-surge deposits are generally thought to represent deposition from turbulent, low-concentration pyroclastic density currents resulting from “ash hurricane,” initial stages of Plinian eruption column collapse, or blast-wave phenomena. Ground layers form by sedimentation from the fluidized head of a pyroclastic flow, and jetted deposits form by explosive jetting from the flow head as a result of heating of ingested air. Theoretical evidence suggests some layer 1 deposits may result from turbulent boundary layers at the base of pyroclastic flows. Deposits formed from this process will have characteristics that depend on local flow conditions and thus may be largely independent of distance from vent. The turbulenit-boundary-layer model may explain the occurrence of surgelike deposits at distances from sources that appear too far for transportation by a low-concentration, turbulent pyroclastic surge.