A single eruption event may result in a combination of surge, flow and fall deposits (Fig. 17.4). Block- and ash-flow deposits lack the ground-surge unit that may be seen at the bottom of scoria-flow and ignimbrite
deposits. Pyroclastic flow units are typically structureless, although they may display some grading, with reverse grading occurring in the lower density pumice and vesiculated scoria fragments and normal grading in the more dense lithic clasts.