like walls with smooth, straight sides, sometimes leading the stream into sharply rectangular courses where the water departs from one set of joints to the other, which extends at right angles. Both the horizontal stratification and the vertical joint planes also affect the outline of the gorge walls, giving rise to some striking and picturesque effects.
Still another feature prominently illustrated in these gorges is the pot-hole, formed where the cascading waters bore into the stream bed and. grinding pebbles about in the swirling waters, form large, deep pools (Fig. 10), adding interesting variety to the scenery of the gorges,
- For further description of these phenomena see Tarr, Bull. Amer. Geog. Soc., Vol. XXXVIL, 1905, pp. 193-212.