Concentrated lahars moving as debris flow are termed volcanic debris flows (VDFs). The unstratified deposits of individual VDFs commonly contain particles ranging from clay size to boulders and may contain blocks 10 m or more in diameter. Deposit volumes can exceed 1 km3. Relatively small amounts of clay-size particles in VDF slurries play important roles in the mechanics of sediment transport and deposition, thus two categories of VDF deposits are recognized—those that are clay rich (≥ 5 weight percent clay or about 2 volume percent) and those that are clay poor (< 5 weight percent). VDF deposits are common in volcanic sedimentary sequences, primarily at the bases of intermediate-composition volcanic edifices and in downstream river valleys. Clay-rich VDF deposits may be emplaced more than 100 km downstream of source. Their key characteristics include uniform color and sedimentary texture, extremely poor sorting, notably firm/hard consistencies in outcrop, bubble vesicles in matrix material, mainly normal grading, and intergranular voids found only at deposit margins. Clay-poor deposits are similar but more commonly found relatively close to source and exhibit very poor sorting, sandier matrix textures, variable grading of all types, slightly less firm/hard consistencies, and crude rounding and pockmarking of boulders.