Rock beds thousands of feet thick were laid down in northern and central New Mexico during Late Cretaceous time, whereas most of the southern part of the state was above sea level and was being eroded by tireless winds and streams. The shorelines made parallel northwest-trending bands across the state. These are now marked by beach sands, some of which are speckled by black minerals, high in rare elements titanium, niobium, and zirconium. Northwestern and central New Mexico was a battleground of the land and sea, with the beaches advancing and retreating fifty or a hundred miles during an instant of geologic time. Stream sands and coal beds lie landward from the beach sands which, in turn, mingle seaward with black limy shales that were flushed into the seas. The lowest of these rocks is the Dakota Sandstone—famous as an artesian aquifer in the High Plains areas of states to the northeast—Colorado, the Dakotas, Nebraska, Kansas, Wyoming, and Montana. Above is the black Mancos Shale, which in turn is overlain by the Mesaverde Group.
The transitions from coal swamps and stream sands to beach deposits and then into marine black shales is characteristic of the Mesaverde in northwestern and north-central New Mexico. To the northeast, beds of the same age were laid down in an extensive muddy sea that stretched far to the east; the Pierre Shale and Niobrara chalky limestone that underlie the plains northeast of Las Vegas are typical. The cliff-forming sandstones and coal beds near Gallup are part of the Mesaverde Group and rim the entire San Juan Basin. Above are similar rocks such as the Kirtland Shale, Pictured Cliffs Sandstone, and Fruitland Formation that underlie valleys cut in the shales and cliffs carved from the sandstones in the northwest corner of New Mexico near Farmington.
Toward the end of the Cretaceous, the Laramide “revolution” began, and New Mexico along with most of North America emerged from beneath the seas, to be high and dry to the present. The revolution, an extensive upheaval of the earth’s crust, saw uplift of the San Juan Mountains area in southwestern Colorado and large volcanoes spouting fire and ashes nearby. Fragments of the eroded mountains and debris from these andesitic volcanoes were flushed southward by streams and steam to settle as thick piles of mud, sandstone, and conglomerate, the McDermott and Animas formations in the San Juan Basin. The last moment of Cretaceous time, if we could be so precise, was ushered out almost unnoticed—with mountains rising to the north and the andesitic-quartz detritus being laid down to the south in the upper beds of the Animas Formation.
Similarly, mountains arose during Late Cretaceous time in north-central New Mexico and south-central Colorado, about on the site of the present-day Sangre de Cristo range northeast of Taos, and shed erosional gravels and muds into the Raton Basin area. Alluvial fan gravels and sands grade eastward into dark muds and coals laid down in swamps and on floodplains. These rocks now cap the rugged mesas seen northwest of the Santa Fe Railway from Raton southward—the cliff-forming Trinidad Sandstone and the dark siltstones, sandstones, black shales, and coal beds of the Vermejo and Raton formations. The Kaiser Steel Corporation mine near Koehler extracts coal from these beds. Again, the exact end of the Cretaceous is marked only by some obscure boundary between beds, in that area within the Raton Formation.
During the Cretaceous Period, the deciduous trees—such as the oak, maple, poplar, and elm that dominate today’s flora—became common. The covered-seed plants, the angiosperms, are the most notable of the Cretaceous plants, but the development of the modern floras was an antecedent to the great expansion of mammals and birds during the following Cenozoic Era. Reptiles ruled the earth, led by the dinosaurs ([fig. 10]) and their distinctive group, the horned large-skulled ceratopsians such as Triceratops. The small, hairy, warm-blooded mammals were still insignificant creatures that ran from their huge dinosaur lords, but they ate reptile eggs, and so excelled the sluggish reptiles in mental and physical activity that they adapted swiftly to the changing environments of the Laramide revolution—and became dominant as the pea-brained reptiles were unable to stand the changes.
The shallow seas of the Cretaceous swarmed with invertebrate life; foraminifers (unicellular protozoans) in uncountable billions make up large parts of the chalky limestones. Mollusks, particularly clams like oysters and the heavy ribbed Inoceramus, and complexly sutured cephalopods, the ammonites, as well as the internal-shelled belemnoids (that look like cigars), were most numerous among larger marine animals. Reef builders in southwestern New Mexico were the peculiarly corallike clams, the rudistids. Widespread warm humid climates seem to have prevailed throughout the state during most of the Cretaceous.
LANDSCAPES AND MINERAL RESOURCES
Thus as the Cretaceous seas withdrew from New Mexico, the Cenozoic Era dawned, and never again have marine waters shaped the landscapes. The rocks, Precambrian, Paleozoic, Mesozoic, and Cenozoic, and their ancient movements determine New Mexico’s spectacular landscapes. They tell tales of the endless war between erosion and hard rock, show the deposition of sediments, their uplift, and their eventual destruction. The result is striking scenery—volcanic mountains, as Mount Taylor, Sierra Blanca, Valle Grande—fresh lava flows near Carrizozo and Grants—volcanic necks like Shiprock and Cabezon—White Sands, the work of the wind—El Morro, Enchanted Mesa, Acoma, and badlands near Santa Fe, the result of weathering and erosion—great fault-line escarpments of the Sandia, Manzano, San Andres, and Sacramento mountains—the work of underground waters at Carlsbad Caverns. And man adds his erosive powers—the huge open-pit copper mine at Santa Rita and countless excavations for rock to build homes, to straighten highways.
Figure 10. Make way for ’is lordship, the Dinosaur!