The northern side of Box Hill (corresponding to the dip slope of the cuesta of the North Downs) is cut by six combes, also created by Ice Age watercourses. The westernmost of these combes (between the Burford Spur and Lodge Hill) is climbed by the Zig Zag Road. There is a small reservoir to the west of High Ashurst at the base of the Bullen Wood combe, which is not accessible to the public. All six combes contained tributaries of the stream that created the Headley Valley, which joined the River Mole near Fredley, Mickleham.