Cummings continued his explorations into Arizona, where he discovered Betatakin, Inscription House, and other famed cliff dwellings south and east of Navajo Mountain.

His last field work was at Kinishba, a great ruin on the Apache reservation, excavation of which he pursued annually from 1931 through 1939 with student assistants majoring in anthropology.

In 1930, Cummings, then Dean of the University of Arizona Archaeological Department, led an expedition "160 miles south of the [Mexican] border" where Cummings expected to find giant skeletons after having apparently previously found one male and two female skeletons each over 8 feet tall with heads over 1 foot long in a "burial ground to the 'cyclops'".

Cummings was the first to discover Pleistocene man in southern Arizona and his discoveries led to the eventual recognition of the Cochise culture that has been dated to before 6,000 B.C.

Cummings was co-head of the US party attempting to be the first Americans to visit this landmark, along with William B. Douglass, Examiner of Surveys for the United States General Land Office.