Driving along I-25 between Walsenburg, Colorado, and Raton, New Mexico, one enjoys spectacular scenery, including lofty peaks, high mesas and rugged canyons. One would certainly not imagine that this area was once a basin!
Yet, back in the Cretaceous, some 100 million years ago, when Tyrannosaurus rex and his cohorts roamed the planet and a broad seaway stretched from Texas to western Canada, a geologic basin extended across this region, tilted upward to the west; marine shale, known as Pierre Shale, covered the floor of the basin, deposited by the Cretaceous Sea. As the Rockies began to rise and the sea retreated to the south and east, coastal sands settled on the shale and have since hardened into the sandstone of the Trinidad Formation. Then, toward the end of the Cretaceous, a delta ecosystem spread across the region, destined to yield the shales, siltstones, sandstones and coal seams of the Vermejo Formation; coal has been mined from this formation since the 1800s and methane has been retrieved since the 1980s. As the Cretaceous gave way to the Tertiary Period, alluvial debris from the rising mountains covered the area, the source of conglomerates that now comprise the Raton Formation; harboring pockets of coal, this Formation is famous for its exposure of the K-T boundary, within which a high level of iridium supports the theory that a catastrophic meteor impact ended the Cretaceous and wiped out the dinosaurs. Finally, during the Paleocene, alluvial sands and muds of what is now the Poison Canyon Formation covered any remaining evidence of the underlying geologic basin.
Since then, in concert with continued uplift along the Rocky Mountain corridor, granite intrusions pushed up into the sediments, later to be uncovered by erosion to reveal the spectacular Spanish Peaks of southern Colorado, complete with Eocene-Oligocene lava dikes that formed as these massive plutons rose. In the late Miocene, about 9 million years ago, the Raton-Clayton Volcanic Field developed across northeast New Mexico, spreading thick pools of basalt across what is now the Colorado-New Mexico line. This hard cap of basalt protected the underlying Tertiary and Cretaceous sediments, yielding a chain of high mesas that stretch from the base of the Rockies to the Oklahoma Panhandle, separating the watersheds of the Arkansas and Cimarron Rivers.
