The Timberlake Ranch area is located on the sloping eroded flank of an uplift called the Zuni mountains. The uplift commenced about 70 million years ago, and erosion, in that time has removed in some areas, as much as 11,000 feet of sediments that used to be there. The erosion has exposed the formerly buried rocks and we can see them laid out on the surface today, in sequence, surrounding the core of the Zuni mountains; the oldest rocks in the core area near Post Office Flat and McGaffey, and younger rocks exposed surrounding the core. Timberlake Ranch lies in a forested valley, which surrounds the core of the Zuni mountains, and is surfaced with Triassic age rocks. The name Triassic refers to the time period which produced the fossil life forms found in the rocks, here, as well as in all the other Triassic rocks found on Earth. The formative processes trapped whatever types of living things were there at the time, and that gave the rock a time signature by which it can be dated relative to the progression of living things over time. Many Timberlake residents have found petrified wood pieces scattered over the hills, but there are leaf, and rarely, reptile fossils in the Triassic rocks also. The oldest Triassic rocks are up the slopes toward the north and northeast of the Old Ranch House, on the slope leading up into the forested areas toward McGaffey. The younger Triassic rocks are found lower on the same slopes and across the Timberlake valley, with the youngest being on the reddish slopes in the Clo-Chin-Toh Ranch foothills along and to the east of the Timberlake road. This youngest layer is called the Rock Point member and is described as a reddish orange to reddish brown siltstone with a greenish gray sandstone interbedded within. The top 50 feet contains red mudstone and grayish white silty sandstone. It forms slopes broken locally by few rounded ledges. The origin of this layer was the bottom of a lagoon or floodplain. These reddish, fine grain sediments, were laid as outwash from nearby rivers. This layer is as much as 500 feet thick in the Zuni area, averages 200 feet in the Timberlake area, then thins, and disappears south of Gallup. The slope of this layer is about 3 degrees, that is, the angle of the sediment beds as compared to horizontal, and were tilted by the force of the Zuni uplift. Crossing the modern day floodplain above the lake, which consists of sediments washed out from the Triassic rocks of the valley, and entering over the Timberlake cattle guard, the hills to the right and ahead are in the next older layer of Triassic rocks, called the Sonsela sandstone. The Sonsela sandstone does contain sandstone strata, but the majority of the layer is siltstone and mudstones. This explains why Timberlake Ranch is in a valley. Siltstones and mudstones both are weakly cemented, and relatively easy to erode, so they produce a flattish topography of alluvial beds and rolling hills. They are described as grayish red to pale reddish brown and purple flat bedded mudstone, siltstone, and sandy siltstone. They were formed from river deposits where the water was moving faster than in the previous Rock Point member. This layer was measured at about 900 ft. thick in the Thoreau area. As you move up the slopes to the northeast, and into the northern part of Timberlake Ranch, the Sonsela sandstone beds start to become evident as ledges and ridges separated by grassy alluvial valleys and meadows. They are described as light gray to yellowish brown channel sandstones separated by layers of bluish gray mudstone and siltstone. This layer has been measured to be about 164 ft at Ft. Defiance, Arizona. Sandstone by definition has larger grains making up the rock, compared to a siltstone or a mudstone. Also, sandstone tends to be made more of sand grains while siltstone and mudstones have more clay in their structure. These differences come from the environments that existed during the laying down of the sediments. Generally, sandstone formation means higher velocity environments, while siltstones come from slower velocity, and claystones and mudstones are produced in the slowest moving environments. Moving further up slope to the northern borders of the Ranch, brings the hiker into older Triassic layers, first, the Petrified Forest member. This member was named in Arizona at the Petrified Forest, and has the same characteristics here in Timberlake Ranch. It is described as variegated claystone, siltstone, and minor amounts of sandstone and petrified wood. It may contain small channel type uranium deposits. Secondly, moving farther north, and interbedded with the Petrified Forest member, is the Monitor Butte member, described as fine grained crossbedded sandstones, and a siltstone as much as 390 ft thick. The Triassic layer which is oldest and furthest up the hill, is only occasionally present, and called the Shinarump member which is described as yellowish orange to yellowish gray and grayish purple, fine to coarse grained quartz congomerate sandstone deposited by flowing rivers and streams. These sandy layers have been measured to slope at 5 to 7 degrees from the horizontal. Continuing up the foothills into the upper reaches of the Zuni mountains, brings the hiker to older rock of the Permian time which are not present in the Timberlake Ranch. This listing of sedimentary rocks lays out the environment at the time, as a low elevation floodplain that was crossed by rivers and streams, some of them vigorous in their flow, while others were slower and sometimes stagnant. There were lagoons and areas where floodwaters overflowed and ponded before soaking in. The petrified wood originated far upstream and floated downstream to be deposited in wide bends of the rivers. Waterlogged wood sank into the muds, and uranium, quartz, and other minerals were soaked into the wood replacing the organic structures. These rock layers were deeply buried by other sediments in time, then with the uplift of the Zuni mountains, the rock layers were tilted, erosion slowly reversed the process, the Triassic layers were exposed, and today are slowly washing and blowing away.