This research work was conducted in an attempt to assess the remaining gold resources in the remnant Early Tertiary river deposits that were left untouched by hydraulic mining activity between the late 19th Century and the early 20th. Much of these remaining gold reserves contained in the ancient river sediments lie below considerable overburden including the Mehrten Formation volcaniclastic sediments (andesitic) that form resistant ridges in the Sierra Nevada Foothills.
What is interesting about this document is that the descriptions of the rock units give subtle clues into the mineralogical distinctions between the “lower or channel” gravels and the “upper or bench” gravels. More recent work has shown that the lower or channel gravels are kaolinitic while the “upper or bench” gravels become distinctly smectitic higher in the section (see Wood and Glasmann, 2013). While Yeend described no specific matrix mineralogical differences between the lower and upper gravel units, he described the clay beds of the upper unit as green which is an idiochromatic color of the smectitic clay beds in the upper unit (see descriptions of Pask and Turner, 1952 for their “upper Ione” member clay beds).
Yeend, Warren E., 1974, Gold-Bearing Gravels of the Ancestral Yuba River, Sierra Nevada, California; Professional Paper 772, USGS, Washington, D.C.