TRnews editors:
Roger
Irwin
Tim Amsden
Contact
the editors at:
Advisory
Board
Genghis & Sylvia Kahn
Alfred
E. Neuman
We welcome contributions. Please contact the local interest editors.
TRIASSIC
ROCKS OF TIMBERLAKE RANCH
by Bob Schafer
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.
RIVER AND LAKE by Tim Amsden (Published in the Timberlake Times November, 2000)
The intermittent Zuni River twists and turns down hills and through crevasses, tumbles through the upper and lower waterfalls, and runs finally across the valley floor into Ramah Lake. The amount of water it collects from 30,000 acres of the Zuni Mountains varies widely, depending somewhat on summer rainfall but mostly on melting winter snow pack. Right now the lake is very low, due to two winters with very little snowfall.
The lake is normally about two miles long, but it changes greatly from year to year and season to season. In a very wet year it may extend up to 3 miles in length - in a dry year or season (during heavy irrigation), it can dwindle down to a fraction of that. The lake's maximum water depth is something less than 95 feet at the dam, depending upon how much siltation has occurred over the years.
We don't know how old the lake is; there is some indication that the Navajos had a small dam at the location of the current dam long before white people arrived. We do know that Ernest Tietjen, with 20 hired Navajo men, one yoke of oxen, and a scraper made from Fort Wingate scrap, created the first dam of any size. We also know that since 1880, the dam has washed out and been rebuilt three or four times; and has been enlarged, improved, and maintained time after time, at sometimes heroic expense and labor, by the Mormon people who use the water for irrigation.
The fish of the lake are regulated by the New Mexico Fish and Game Department, who regularly stock it with rainbow trout. There have been proposals to expand the lake and the fishery by raising the dam, but because of the concerns of the State Engineer and the expense, it is highly unlikely.
Whether covered with ice and snow or surrounded with the bright leaves of the cottonwoods and willows, Ramah Lake is one of the most picturesque places in this most picturesque land. It's a wonderful place to fish, hike or just enjoy the ducks, great blue herons, eagles, and occasional migrating pelicans and ospreys. Next time you are there looking at the towering sandstone cliffs, think about all the people that helped to create and maintain it. Now it's partly our lake to care for; if you see a beer can or old bait container someone tossed out, pick it up and carry it away.
The Red Cliffs of Timberlake
Entering Timberlake for the first time, one is awestruck by the majestic beauty of the Zuni sandstone cliffs that border Timberlake to the west.
The cliffs have been referred to as hogbacks, but actually are cuestas. These asymmetrical ridges tend to form when there is a tilted and eroded series of alternating resistant and soft beds. One side of a cuesta has a long, gentle slope determined by the angle of inclination of the resistant bed; the other side is a steep escarpment formed at the erosional edge of the resistant bed where it is undercut by erosion of a weaker bed beneath. Cuestas differ from hogbacks in that the resistant rock layers forming the hogback intersect the surface at much steeper angles, creating steep slopes on both sides of the intrusion.
Ramah
lies between 2 cuestas that circle the Zuni Mountains. A Cretaceous cuesta to
the south and a Jurassic one to the north - Ramah Lake lies in the gorge of the
carved salmon-striped Zuni sandstone cuesta.
Deposition
of the varying layers of Zuni sandstone occurred during the Mesozoic Era - specifically
the Jurassic period and the Cretaceous period. Northern New Mexico was covered
by a sea of sand 144-208 million years ago (Jurassic period) similar to the Sahara
desert of today. Briefly a vast sea covered parts of New Mexico 66-144 million
years ago (Cretaceous period) as North America broke away from Europe and started
to drift westward.
So why is there varying pink and cream-colored layers to the Zuni sandstone and what was deposited by water or wind? The reddish color is due to tiny grains of hematite - an iron oxide that we normally think of as rust. Iron oxides in different amounts impart many shades of color to sedimentary rock ranging from yellow, pale pink to deep red tones.
Capping the cliffs of the Zuni sandstone is a hard resistant layer called Dakota sandstone. It is the oldest Cretaceous sedimentary layer deposited along the western shore of an advancing Cretaceous sea. The Dakota sandstone is about 30 millions years younger than the Zuni sandstone and was deposited on its eroded upper surface.
One can now envision hundreds and hundreds of years of blowing sand (some mixed with hematite) and then a sandy layer deposited by the Cretaceous sea, its sand grains not as well rounded or sorted as those of the wind deposited Zuni sandstone. So the next time there is a gorgeous sunrise or sunset, a July rainstorm or a December snowfall, look at our Zuni sandstone cuesta and remember how long it took to give us Timberlakers a breathless moment.
Roadside Geology
of New Mexico, Holka Chronic, 1987
Understanding Earth, 3rd Edition, Press
& Siever
Earth, 3rd Edition, Press & Siever