How Rivers Carve Mountains and Widen Valleys
River-bank depth, worn strata, and why confluences cut acute mountain angles
This transmitted-light plate collects Leonardo's rules on rivers wearing away mountains. He states that the water is deepest at the middle of a concave bank and shallowest opposite it at the convex bank, and that stratified stones are worn most at the front of their layers, leaving mountain flanks rough where undermined rock breaks and collapses. He then reasons that valleys widen most at the confluence of two rivers, because an acute mountain angle caught between two currents is consumed faster than an obtuse one, with a lettered figure marking the acute angle a and obtuse angle b. Finally he notes that broad-summited mountains yield more springwater than sharp peaks, since they absorb more rain.
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Deepest water at the concave bank
The greatest depth of water lies in the middle of the concave bank struck by the current, and the least depth always in the middle of the convex bank it leaves, so the two lie directly opposite one another.
Stratified stone worn at the front, breaking the flanks
Layered stones are worn most by the water at the front of their layers rather than their sides. Once their fronts are eaten away they can no longer bear the rock above, which breaks and collapses, leaving the mountain flanks rough and harsh, unlike the smooth riverbed.
Confluences and the acute mountain angle a, b
Valleys widen more where two rivers meet than along either river alone, because an angle of mountain caught between two currents is consumed faster than one river can wear a single flank. The figure marks the acute angle a and the obtuse angle b; the more acute the angle, the greater the valley.
Broad-summited mountains yield more water
Mountains with wide plains at their summits pour greater waters at their base, because they absorb more rainwater than sharp, cutting peaks, from which rain quickly runs off, and they hold a greater sum of water-veins swollen by the rain.