A reprise: The Grand Canyon National Monument (number 2) that President Hoover established in 1932 recognized the impact & remnants of volcanic action on the Canyon, as well as the different and startling configuration of the Canyon itself -- a section where geologic actions had brought a younger set of rocks to river level. NPS interest was definitely stimulated by features near the river. In the same period, there was considerable thought being given as to how to classify and dispose of or administer the public lands of the Arizona Strip west of Kanab Creek. The character of these lands, away from the river, moved from evidence of the volcanic-drowned nature of Toroweap Vally with its cliffs and old flows to the open, extensive reaches of Kanab Plateau, running on toward the Utah border.
That being a time when NPS officials at GCNP were still learning what the Grand Canyon actually was, it can not be too surprising that GCNM2 was a big fella, taking in a big swath of the rolling, above-rim lands as well as those in and near the canyon. More detailed study to come up with the most appropriate boundary was urgent because of the complaints of the dozen or so ranchers who ran stock there -- a task that Minor Tillotson, first as Superintendent and then Regional Director, dedicated himself to over the next 20 years. He got lots of advice from the residents of Arizona and Utah north of the Grand Canyon who were used to the Arizona Strip, all the way down to the Colorado, being their (free) grazing range. Negotiations through the 1930's did finally lead to a boundary brought closer to the river, but which still included lands to protect approaches to the side canyons, e.g. Tuckup, those flatter lands of the plateau that graziers wanted to continue to use. (The Grand Canyon National Monument #2 story in detail is told in several posts, Jan-Mar 2012.)