Elmer Cook, a local rancher and resident of the area, first discovered fossil horse remains in the late 1920s.
He reported the find to Dr. Harold T. Stearns of the United States Geological Survey, who in turn brought it to the attention of Dr. James W. Gidley of the Smithsonian Institution.
Besides the fossil horse, Equus simplicidens, other large vertebrates collected from the quarry include an antelope, a camel and a peccary.
Small mammalian fossil vertebrate genera include hare, weasel, gopher, vole and shrew.
Also represented are fossil woodland birds, waterfowl, snakes, turtles, lizards, frogs, toads, salamanders and a variety of fish.
The western Snake River Plain consists of rift basin sediments that accumulated during the Miocene through Pleistocene Epochs.
These clastic sedimentary packages, interbedded with basaltic flows, pyroclastic tephra and silicic volcanic ashes, have a cumulative thickness of 1524 meters (5000 feet), span about 10.5 Ma, and comprise the seven formations of the Idaho Group.
In the Hagerman Fossil Beds National Monument, the Glenns Ferry Formation has a maximum thickness of approximately 183 meters (600 feet) and spans the interval 4.5 to 3.0 Ma.
Lithostratigraphically, the Hagerman Horse Quarry is located within the upper member of the Glenns Ferry Formation and lies 9.5 meters below the contact with the overlying Tuana Gravel.
The Tuana Gravel is overlain by a caliche, thought to have formed during a Pleistocene interglacial period, and several feet of recent soils (Richmond and others 2002).
Dr. James W. Gidley (1930) originally interpreted the Smithsonian fossiliferous red sandstone bed to have been deposited in a bog or water hole.
Basing their conclusions on historical information, photographs, and collection samples, Akersten and Thompson (1992) proposed the fossil accumulation was the result of a single flood event, which trapped and killed the horses, then transported their carcasses.
All three quarry sandstones are interpreted to be ephemeral braided fluvial channel systems that were deposited in the Snake River Plain graben (Richmond and others, 2002).
The Phase II drought on the Snake River Plain resulted in a mass mortality of Equus simplicidens in addition to many other macro- and microvertebrates in the quarry area.