“…The valleys incised by such streams were bounded by slopes that develop from below (e.g. Penck, 1924;Bremer, 1983;Twidale and Milnes, 1983), because water gravitates to lower slopes and the piedmont, where in consequence, weathering and erosion are most pronounced. This caused the undermining and steepening, regrading and recession of hill slopes, particularly those capped by a duricrust or a resistant stratum, and as appreciated by Holmes (1918), .…”