Drag resistance developed between a pyroclastic flow and the ground results in a transitional zone of low velocity between the maximum velocity of the flow and the stationary ground. Fragments of all sizes within the turbulent flow travel irregular paths and therefore enter the reduced velocity zone at random and are deposited together irrespective of size. This process results in poorly sorted deposits with systematic vertical variations in mineralogy which are dependent upon variations in the original magma chamber and with lateral variation which are dependent upon the gradual energy losses of the main flow. Mineral variations in one unit of a compound cooling unit, the Picture Gorge ignimbrite, from the John Day Formation in eastern Oregon are explained on the basis of the above model.