Most U-shaped valleys—valleys with a parabolic cross-section—are glacial troughs. Brook et al. (2008) Geomorph. 97, 1–2 find that valley cross-profiles become U-shaped with c. 400,000–600,000 years of glacial occupancy. When no longer supported by ice, the steep sides of U-shaped valleys are prone to rock fall (Blair (1994) Mountain Res. & Dev. 14). Valleys with this form are also encountered in non-glaciated chalk topography.