Geology Landscapes of Valencia County: Many changes have been made in New Mexico

What did New Mexico look like in the time of dinosaurs (66 to 251 million years ago)?

Many changes to the geography of our state occurred during that long time span.

During the Triassic Period, the first of three major time periods in the Mesozoic, New Mexico was a lowland river system with Mississippi-size rivers flowing from the southeast to the northwest. One of the earliest dinosaurs, Coelophysis, thrived in this seasonally arid climate, as did cycads, ferns, and large amphibians.

During the Jurassic Period, three successive environments included a great expanse of sand similar to today’s Sahara, a large salty inland sea, followed by warm tropical floodplains, when in Late Jurassic times huge rivers flowed to the east and formed muddy floodplains. Dinosaurs, such as gigantic Seismosaurus and carnivorous Allosaurus, roamed the land.

During the Cretaceous, New Mexico was split by a seacoast, with sea (the Western Interior Seaway) to the east and land to the west, as shown on the figure from cretaceousatlas.org. Huge rivers flowed from the west to a coastline hugged by dense jungles and coal-forming swamps. Dinosaurs roamed the area.

Much of the Cretaceous shoreline swamp and forest vegetation has been transformed into the abundant coal and gas deposits found in northwestern New Mexico. The muddy sea floor to the east filled with oysters and other mollusks. Interior seaway animals included sharks and predatory marine reptiles such as plesiosaurs and mosasaurs. Ammonites and other sea creatures swam in the warm shallow waters.

A large meteorite hit the earth 66 million years ago, killing all non-bird dinosaurs and many other species.

The rocks of the Mesozoic Era are visible today as colorful layered sandstone, siltstone, shale, and limestone shaped by erosion into the mesas and buttes of northern and western New Mexico. Just west of Raton, N.M., an easy hike leads to the exact earth layer that marks the catastrophic end to the dinosaur era.

Visit the New Mexico Museum of Natural History & Science in Albuquerque for exciting displays of life in the Mesozoic.

(Paul Parmentier, a certified professional geologist retired from California and living in Los Lunas, shares the rich geologic features in Valencia County. He earned a Bachelor of Science degree in geology from Belgium and a master’s degree in geochemistry from Japan. The Geology Landscapes of Valencia County are featured monthly.)