R.C.Moore, the geologist on the 1923 USGS survey trip led by Birdseye, added a 46-page Appendix B to La Rue's 1925 monograph (see two earlier entries). In a half-century, no geologist except for Powell and Gilbert had had the opportunity to study the formations along the river for any distance. And none had been concerned with the problems of flood control and water power. So he worked to study all geologic features, giving special attention to river control and development. His narrative makes a very special guide for a river trip, in this instance, imagined on my part. He described each division --Marble Gorge, Kaibab, Kanab, Uinkaret, Shivwits-- in a linear fashion, stopping for a detailed reporting of each damsite, summing up its qualities; looking for good, strong foundations and abutments, the likelihood for leakage around the dam, and whether there is nearby good rock for construction material. A qualitative assessment, but one that sounds very authoritative. Here he is* on the Specter Chasm damsites, ~mile 130:
They are in the highest narrow section of the Middle Granite Gorge, with walls composed of hard dark reddish-brown to bluish-black schistose quartzite and quartzitic schist, with some black amphibolite, somewhat extensively injected with medium to coarse grained pink granite pegmatite in irregular veins and with thin veins of milky quartz. [It is to eat.]
The strike is essentially parallel to the course of the river. The rocks are very hard, being broken by the hammer only with considerable difficulty. This hardness and the very dense, impermeable texture make the rock very satisfactory for the foundations and abutments, though they will increase the cost of drilling and quarrying. The schistose structure is not so well developed as to be of much aid in working the rock. The dark rocks are undoubtedly an indurated, somewhat metamorphosed sedimentary series which has been closely folded, squeezed, and in places more or less broken.
The right wall is rather smoother than the left, for the planes of schistosity often define the slope. Exposures of the pink granitic intrusive rocks are less conspicuous. The left wall is steep and jaggedly rough, the slope cutting across the schistose structure. Intrusive pink granite pegmatite is abundant, and extends for considerable distances subhorizontally.
The Archean rocks are capped by a 100' cliff of Tapeats sandstone, medium to coarse grained, irregularly cross-bedded, and hard.
The river flow here is quiet and rather even, filling the canyon. The estimated depth to bedrock is 30 to 50 feet.
The dam site walls are so dense and relatively so unshattered by joints or fractures that in spite of the trend of the schist, they are entirely satisfactory as a foundation, and believed to be impermeable to water under pressure. (This photo is plate XXXII in La Rue, 1925.)
The quartzite of the walls is more than sufficiently dense and hard for use as concrete aggregate. It is rather difficult to get at and expensive to crush. The higher walls furnish an abundance of suitable, fairly easily accessible, and readily quarried material for concrete. The massive brown hard dolomite in the Bright Angel will furnish large quantities most easily obtained, as this bed forms extensive benches with gentle slopes and little cover. The Muav and Redwall limestones are satisfactory, but higher and more difficult to work.
Sand is most readily obtainable by crushing the unlimited quantities of Tapeats sandstone, which has grains that are medium to very coarse and gritty, and subangular. The cementing material is not such as to make crushing expensive.
*I have mostly used his language, but abridged the selection; therefore, it is not a complete, direct quotation (cf. pp.151-3 of La Rue's 1925 "Water power and flood control of the Colorado …").