The mountains, the plains, the rivers, and the lakes all are transitory
features of the landscape, created during the recent part of the
Cenozoic Era, and all doomed to destruction in the near future,
geologically speaking. Even so, some of the plains and mountains may
predate man’s evolution onto the earth scene.
Gone were the late Mesozoic seas; never again during our lifetime nor
the lifetime of many future generations will marine waters roll over New
Mexico, and sea-spawned creatures rule. For the first two thirds of the
Cenozoic, the Paleogene Period (25 to 70 m.y. ago), New Mexico suffered
from the dying effects of the great Laramide upheaval of the earth’s
crust. Large areas were exposed to the harsh erosion of stream and wind.
The landscape looked as parts of southwestern New Mexico do today—tall
rugged mountain ranges scattered in isolated patches amid wide gravelly
plains. But the climate was more humid, and while no large through
rivers are known, local great swamps and lakes lay on the plains in
debris-trapping lowlands.
[Illustration: Figure 1. Table of geologic time]
_Scale of geologic time
in millions of years_
_Era_ _Rocks_ _Dominant life_
_Geologic age_
70 CENOZOIC
Neogene
Pleistocene Bandelier volcanic ash, Man
basalts, sand dunes, river
gravels, glacial & lake beds.
1 my
Pliocene Volcanic rocks of Mt. Mammals
Taylor, early Valle Grande &
Gila region; Santa Fe, Gila,
and Ogallala Fms.
11 my
Miocene
25 my
Paleogene
Oligocene Datil, Espinaso, and other
volcanic rocks.
40 my
Eocene Baca, Animas, Nacimiento,
San Jose, Raton, Poison
Canyon, Galisteo El Rito,
Blanco Basin, & Cub Mountain
Fms.
60 my
Paleocene
160 MESOZOIC
Cretaceous Upper sandstone, shale, & Dinosaurs
coal. Mesaverde, Pierre, &
Niobrara, Dakota Ss. and
Mancos Sh.
Volcanic rocks, limestone,
sandstone, and conglomerate.
135 my
Jurassic Morrison Fm., Summerville
Fm., Zuni Ss., Todilto Ls. &
gypsum, Entrada Ss.
180 my
Triassic Wingate Ss., Dockum Fm.,
Chinle Fm., Santa Rosa Ss.,
Moenkopi Fm.
370 PALEOZOIC
Permian Rustler Dolomite; redbeds Amphibians
Castile gypsum, Salado Salt.
Artesia Grp.—Capitan reef
San Andres Ls.—Goat Seep reef
Glorieta Sandstone
Yeso Fm.—Bone Spring Fm.
Abo Redbeds—Hueco Ls.
280 my
Pennsylvanian Mostly limestone; beds of
shale & sandstone; lenses of
gypsum, salt, and coal.
310 my
Mississippian Helms & Paradise Fms. Fish
Rancheria Ls.
Lake Valley & Escabrosa Lss.
Arroyo Penasco & Tererro Fms.
345 my
Devonian Percha Shale, Ouray Ls.
northern dol., ss, and sh.
erosion
400 my
Silurian Fusselman Dolomite
erosion
425 my
Ordovician Montoya Dolomite Invertebrates
El Paso Limestone
Bliss
500 my
Cambrian Sandstone
erosion
1500+
PRECAMBRIAN quartzite, gneiss, rhyolite, Simple
andesite, granite, primitive forms
pegmatite, schist,
greenstone.
[Illustration: Figure 2. New Mexico during Paleogene time]
Scenery in north-central New Mexico (fig. 2) may have been similar to
today’s, with mountains in the same general areas as the present-day
Sangre de Cristo, Nacimiento, San Juan, and Brazos ranges.
Coarse-grained gravels were stacked up at the edges of the mountains,
but out in the adjoining lowlands, floodplain sands and varicolored
lake-bed clays settled. Three low areas were “basins” of deposition
where thick masses of sediments accumulated—the Raton and Poison Canyon
formations in the Raton Basin near Raton, the Animas, Nacimiento, and
San Jose formations in the San Juan Basin north and northwest of Cuba,
seen along N.M. Highway 44, with thinner deposits of the El Rito and
Blanco Basin formations to the northeast of Cuba, and the Galisteo
Formation in the Galisteo Basin south and southwest of Santa Fe.
Volcanic rocks, the Espinaso beds, overlie the Galisteo but are not much
younger in age. Reddish rocks of the Galisteo Formation crop out along
U.S. Highway 85 at La Bajada Hill about twenty miles southwest of Santa
Fe. The Sandia Mountains’ area appears to have been a lowland.
Silicified wood, chiefly of pines but with some oak and poplar, is
abundant in the Galisteo Formation. Large logs, up to 6 feet in diameter
and 135 feet long, have been found. In the great swamps of the Raton
Basin, where the climate was much like that of Georgia today, tall
reeds, water lilies, fig trees, palm trees, magnolias, and sycamores
grew in profusion, and contributed to the thick coal beds now mined
there. The early ages of the Cenozoic saw the spectacular rise of the
mammals to dominance over reptiles on land; numerous remains of the
early mammals are found in the Nacimiento and San Jose formations,
including the famous Puerco and Torrejon faunas—as well as many clams,
snails, fish, turtles, crocodiles, snakes, and birds.
Southeastern New Mexico appears to have been relatively level with only
local hills and vast regions of featureless, stagnant but high plains
where erosion slowly ate downward, deposition was slight, and most of
the detritus was carried eastward far beyond the state’s borders. The
redbeds of the Baca Formation were laid down on the north flank of low
mountains that extended intermittently from somewhere near Quemado
toward Socorro. Some ancient hills near present-day Sierra Blanca shed
rock fragments that accumulated near Capitan as the varicolored Cub
Mountain Formation. Deeply eroded uplands northwest of Elephant Butte
Reservoir supplied gravels and sands that mingled with andesitic
volcanic debris as the upper part of the McRae Formation in central
Sierra County. Many of the weathered greenish and purplish volcanic
rocks in southwestern New Mexico were extruded at this time, and beneath
the surface these molten magmas (hot liquefied rocks) cut into older
rocks. Vapors and hot solutions from the magmas are believed to have
emplaced some of New Mexico’s vast ore deposits during this time.
The last phase of the Paleogene Period, about 25 to 40 m.y. ago, was an
earth-shaking time in New Mexico—and the first explosion of an atomic
bomb in 1945 on the Jornada del Muerto between Socorro and Carrizozo was
a relatively low-energy-yield event compared with the late Paleogene
earth movements. Almost the entire southwestern quarter of the state
literally exploded, with volcanic eruptions on a grand scale. These lava
flows, rock breccias, ashes, pumice, and associated intrusives (molten
rocks that did not make it to the surface) form the Datil-Mogollon
plateau—at least 100 miles in diameter—as part of the Datil Formation,
which locally is miles thick, and made up the main mass of many other
ranges near the Mexican border. Sierra Blanca (12,003 feet altitude)
northeast of Alamogordo is a huge, isolated volcanic mass of late
Paleogene age.
[Illustration: Figure 3. East-west cross section of Rio Grande
graben near Santa Fe]
This widespread volcanic activity continued into the Neogene Period
which began about 25 m.y. ago. Rhyolites, pumice, and perlite in the
southwest, as well as in other parts of the state, covered wide areas.
Mount Taylor, towering up to 11,389 feet near Grants and visible on the
western skyline from Albuquerque, is a Neogene volcanic pile, as are
parts of the Sangre de Cristo range northeast of Taos. Shiprock and
Cabezon Peak, landmarks in northwestern New Mexico, are volcanic
necks—the eroded cores of ancient volcanoes.
Valle Grande caldera makes up the center of the Jemez Mountains west of
Los Alamos and is a late Neogene volcanic mass with the central crater
sixteen miles in diameter—one of the world’s largest calderas. Bandelier
National Monument headquarters is within a canyon carved from Valle
Grande’s ashes. Volcanic ash scattered over the western parts of Texas,
Oklahoma, and Kansas was blown from this volcano. Capulin Mountain, east
of Raton, is a huge recent cinder cone and is surrounded by numerous
basaltic lava flows that cap the High Plains from Raton eastward to
Clayton. The very fresh black basalt flows near Carrizozo and in the
valley of Rio San Jose near Grants are probably less than 1000 years
old. Numerous mesas along the Rio Grande Valley from the Colorado line
to El Paso are capped by black basalt flows of late Neogene age.
Many of the present-day mountains were uplifted in early Neogene time,
following the climax of the great volcanic eruptions. This uplifting, in
many instances, took place along one side of huge mountain masses,
forming tilted fault blocks like the Sandia, Manzano, San Andres, and
Sacramento mountains. Rock beds in the Sandia Mountains, for example,
dip to the east, but were uplifted along a west-bounding fault zone—a
huge break in the earth’s crust—as much as four miles! This was an
earth-shaking event! However, the uplifting took place slowly, and
indeed is continuing today as the Albuquerque area, along with the Rio
Grande Valley southward to Socorro, is one of the most active earthquake
areas in the state.
Concurrent with uplift, other blocks of the earth’s crust sank, forming
graben basins which were flooded with rock debris from the adjoining
uplifts. A tremendous irregular graben, now followed by the Rio Grande,
cut north-south across the state. Geologists label it the _Rio Grande
structural depression_ (fig. 3). Mountains on the east are the Sangre de
Cristo, Sandia, Manzano, Los Pinos, Fra Cristobal, and Caballo ranges;
those to the west include the Brazos, Jemez, Ladron, Socorro, Magdalena,
and San Mateo mountains. Within this complex graben, and around the
bordering ranges, the colorful sandstones and siltstones of the Santa Fe
Group were deposited—these red, yellow, orange, and cream rocks are
eroded in many places, such as near Santa Fe, to “badlands”
characteristic of the landscapes along the Rio Grande Valley from
Espanola southward to El Paso. Much brightly tinted silicified wood is
found in these beds, and literally freight-car loads of mammalian
remains have been shipped to museums from outcrops near Espanola.
In the basins amid the mountains of southwestern New Mexico, similar
sands and gravels of the Gila Conglomerate filled low areas. East of the
mountains of central New Mexico that form a north-south chain of ranges
from Raton to Carlsbad, thin gravels of the Ogallala Formation were
dumped onto the western edges of the High Plains. They now cap the
plains as well as make picturesque bluffs east of the Pecos River and
southeast of Tucumcari—the “caprock” of that area. In northwestern New
Mexico, isolated mesas are topped by the Chuska and Bidahochi
formations; similar sands, silts, and clays washed from adjoining
highlands.
The final episodes of landscape formation occurred during the
Pleistocene Epoch, the recent glacial period. Mountain valley glaciers
occupied some of the higher parts of the state, as far southward as
Sierra Blanca; large lakes filled many of the closed basins, such as
those near Estancia and south of Lordsburg; the Carrizozo and Grants
basalt flows were extruded; the final tremendous explosions of Valle
Grande spread volcanic ash over large regions; sands, gravels, and clays
were eroded and deposited by streams and in lakes; and sand dunes were
heaped up in many areas. The glistening white gypsum dunes (fig. 4) of
White Sands National Monument, built up into 50-foot-high mounds
windward of gypsiferous Lake Lucero, are spectacular products of the
wind.
The Rio Grande, in its present valley, probably is only as old as
mid-Pleistocene, born during late uplift of its headwater mountains, the
San Juan and Sangre de Cristo ranges in southern Colorado and northern
New Mexico—initiated by floods of meltwaters from waning mountain
glaciers. Some of the lower terraces (benches) along the Rio Grande are
very young, being dated by radiocarbon methods at 2600 b.p. (before
present). Until shackled by Elephant Butte Dam in 1916, and smaller dams
up and down the valley, the Rio Grande switched its course with every
large springtime flood. Even with these man-made controls, the Rio
carves new channels during floods and covers flooded fields with silt as
the high waters recede.