A sheet of block spreads or lines in a periglacial landscape. Block fields (felsenmeer) develop on slopes under 5° (A. L. Washburn 1979), or under 10° (Caine in White (1976) Quat. Res. 6). See van Steijn et al. (2002) PPG 26, 4. Thompson and Syverson (2006) GSA Abstracts with Progs, 38, 4 maintain that this debris is formed in situ by freeze–thaw. Others suggest that the blocks have ridden down on the top of saturated debris during gelifluction (Creemeens et al. (2005) Geomorph. 70, 1–2).