The Miocene Ellensburg Formation of south-central Washington consists largely of volcaniclastic, fluvial sediments deposited on a gentle eastward-sloping surface off the Cascade volcanic belt. The Ellensburg fluvial systems were periodically choked with texturally immature volcanic detritus and were probably characterized by rapid deposition and aggradation. Pillar structures and pocket structures of water-escape origin are generally uncommon in coarse-grained sediments, but are common in conglomeratic deposits of the Ellensburg. Development of water-escape structures in the Ellensburg and in other coarse-grained units is probably dependent on a combination of variables that includes depositional slope, rates of sediment deposition and aggradation, textural maturity, sediment density(?), and regional seismicity.