_Fox Valley._ Down-warped flows of Picture Gorge Basalt dip toward Fox
Valley from all sides to form a basin (Fig. 12). The valley is eroded
out of ashy beds and gravels of the Mascall Formation which fill the
center of the basin to an estimated depth of 1000-1200 feet. Faults form
parts of the northern and southern borders of the basin. The straight,
timbered, northward-facing steep slope less than a mile southeast of the
viewpoint marks a fault.