These gravels originated during the Eocene in rocks somewhere deep in Nevada beyond today's Mother Lode country, from highlands that are long gone. Most of the gravel was overburden, the gold-bearing zone being at the bottom. Remnants of the auriferous gravel have been well mapped, starting with Waldemar Lindgren's 1911 publication "Tertiary Gravels of the Sierra Nevada of California." This exposure is also part of the railroad's ribbon of land that escaped the hydraulickers. At this spot the Eocene gravels are topped by igneous deposits of the much younger Mehrten Formation, showing that little tectonic activity had occurred in some 45 million years.