The Late Paleozoic
Permian Periods [L. Penn.-Triassic]
(280-260 million years ago)
The Permian Period marks a time of great change in the motion of the Earths plates that affected not only western North America, but all major continental land masses.
During the final 90 million years of the Paleozoic era, collisions between continents occurred on a massive scale as Europe, North and South America, Africa, Antarctica and other continents united to form the supercontinent Pangaea. Continental collisions built high mountains where lowland forests, swamps and coral reefs once flourished. Many of the worlds shallow seas became isolated and dried up. Entire ecosystems were obliterated, resulting in massive extinctions.
Evidence for this plate tectonic upheaval is recorded in the rocks formed during the Permian Period at what was to be LMNRA. Mountains that formed when South America and Africa collided with North America lifted the land so that the shallow seas that had covered the west for much of the previous 300 million years receded. Sediment eroded from the newly uplifted ancestral Rocky Mountains was deposited in this area by a large system of rivers. As the climate became increasingly arid, some of the sediment was transported from the river beds by wind and deposited as large dune fields. Minute quantities of iron in these stream and dune deposits gradually oxidized (rust!), turning them a distinctive brick-red color - geologists call these 'red beds'.
Environmental conditions became increasingly variable as eastward advances and westward retreats of the sea became more frequent. Periodically, arid climates promoted the development of dune fields fringed by shallow, restricted tropical seas, similar to the Arabian peninsula today.
For more detail about this time period at Lake Mead NRA click here.
Earth view 306 million years ago
Earth view 255 million years ago