As you climb this mountain, let me tell you what these Franklins actually are, because their story is written in the rock all around you, and it's a violent and ancient one. This whole region is being slowly, ceaselessly pulled apart. Deep forces in the earth are stretching the crust here, part of a great rip in the continent called the Rio Grande Rift that runs from Colorado down into Mexico. And when the crust gets stretched and thinned like that, it cracks into giant blocks — and some blocks drop down to form valleys, while others get shoved up, tilted, to form mountains. That's exactly what the Franklin Mountains are: a great block of the earth's crust, cracked loose along a fault and heaved up on its side.
And because this block got tilted as it rose, it lifted up and exposed layers of rock in order, like a book fallen open on its spine. Down low you can find some of the very oldest rock in all of Texas — over a billion years old, from a time before there was any complex life on earth at all — and as you go up, younger and younger layers. So this mountain isn't a pile that built up; it's a slab of the deep crust, torn loose and tipped skyward, showing you a billion years of the earth's history in its cut faces. The North Franklin Peak, the high point off to the north, tops out at seven thousand one hundred and ninety-two feet. Not bad for a mountain range that sits inside a city. Climb on up the tilted, ancient block. Roll on.
Photo: Susan Barnum · CC BY-SA 4.0
