Hummocky surface is typical of avalanche deposits.
They may form from the collapse of nearly any kind of rock, but the focus here is upon volcanic debris-flow avalanches that develop from the collapse of parts of volcanoes. This photograph shows a large dacitic block from Mount St. Helens within a matrix of smaller sized and more mafic composition fragments. Erosion has carved into the debris avalanche to provide exposures of individual blocks. Although the blocks are coherent masses, they have been extensively fractured and can be excavated with a shovel. An avalanche typically starts movement on a slide plane, but then begins to disintegrate into separate particles that move against and over one another, with the small ones serving as the matrix that carries the larger particles, although this is not necessary for the movement of avalanches.