Upstream of the dam this material would rapidly accumulate to fill the lake, and downstream it would tend to erode its slopes and base. The often rapid accumulation of unconsolidated pyroclastic material on steep sideslopes tends to be inherently unstable over time; pyroclastic dams may be emplaced by the landsliding of such material into rivers and streams. Pyroclastic material, given sufficient time to consolidate or 'weld' into hard rock, produce assemblages variously classified as ignimbrites, variously brecciated or agglomerated, along with various types of tuffs and volcanic ash, and are mostly of felsic composition.