Scarps

Regarding the Rio Grande Rift, Dr. Phil Goodell emailed: “Yes, the Rio Grande Rift is widening, east to west, maybe 1mm/year, or less. Buy some land and watch it grow! And buy earthquake insurance.”

If you take tomorrow morning’s hike from Hondo Pass to Smuggler’s Gap, you might want to take the time to drive from Hondo Pass south on Magnetic/Alabama Street. Although urbanization hides much of the natural landscaping, you will notice as you pass Zion Lane going south or Stoney Hill Drive going north, places where the hillsides have just dropped off. These are scarps – evidence of more ancient faulting as the Rift rifted and the Franklins grew. You are driving along the Eastern Boundary Fault Zone.

You will see a water tank sitting atop a scarp, the result and evidence of a fault. The “dirt” is the Fort Hancock sediment of just a few million years ago deposited as the ancient Rio Grande (and other mountain streams) filled a vast lake geologists have called Lake Cabeza de Vaca. Movement along the fault resulted in the stair-stepping of the sediments and, when the Rio Grande finally broke south and drained into the Gulf of Mexico about a million years ago, the scarps were exposed revealing the fault lines. Further erosion defines these steep slopes for us today all along I-10 on the west, the bluffs by El Paso High School and along Mesa and Alabama.

Horses, camels, mastodons, giant sloths and other mammals lived along Cabeza de Vaca and their fossilized bones can still be found today.

By the way, as Cabeza de Vaca formed and gave a home for ancient animals 3.6 million years ago or so here in a place where we now live, Australopithecines appeared on the other side of the world and then the first Homo species whose descendants, a couple of million years later, would look up at the top of a rift and marvel at the beauty of the Franklins.