18

temporary base level by the destruction of a rock dam in the river.

There is some suggestion that the dissection of the rock floors may have been aided by slight recurrent uplifts of the Marathon dome. Tilting to the south is suggested on the southeast side of the dome by the anomalous position of upland gravel deposits (fig. 4) and by the capture of an extensive drainage system by lower Maxon Creek (fig. 9). Within the basin also dissection is most active in the central and southern parts, where southward-flowing streams would be accelerated by such an uplift. In the northern undissected part of the basin, which also drains mostly to the south, erosion processes do not seem very active and may have been retarded by an uplift with its center farther south. Most of the features in the basin, however, may be interpreted in the other ways.

In many of the valleys the calys of the lowest surface are now in process of dissection by steep-walled arroyos. According to John Bennett, those near his apiary south of Haymond have been cut since about 1920. According to C. L. Baker, those in Green Valley, southwest of the Marathon Basin, have been cut since about 1890. This cutting is similar to that in New Mexico described