How rivers cut mountains and widen valleys
Water deepest at concave banks; valleys widen at the acute angle between two rivers
The page gathers propositions on how running water shapes riverbeds and mountains, with small curling sketches of currents in the right margin. Water is deepest in the middle of the concave bank it strikes and shallowest opposite it, at the convex bank; layered stone is worn most at the front of its layers, and once undermined it breaks and collapses into rough rubble. Where rivers cut through mountains the valley widens most at their confluence, and the more acute the mountain angle caught between two streams (labelled a, against the obtuse angle b) the faster it is worn away. Mountains with broad, flat summits yield more water at their base because rain soaks in rather than running off.
On this page
Water deepest at the concave bank it strikes
The greatest depth of water lies in the middle of the concavity of the bank it strikes, and the least depth in the middle of the convex bank it leaves, the two always lying opposite one another.
Undermined layered rock breaks and collapses
Layered stones are worn most at the fronts of their layers; after long wearing they can no longer bear the stone resting above, so they break and fall, and the broken remainder is left rough and harsh where rivers have cut the mountainside.
Valleys widen at the acute mountain angle between two rivers
In the margin figure a is the acute angle of the mountain caught between two rivers and b the obtuse. Two rivers wear away an acute angle between them far more quickly than a single river wears one mountain side, so the valley opens widest at their junction.