What can rocks tell us?

Ancient weather, creatures, and adventures in time

I’ve traveled from the youngest igneous rocks in California on Lassen Peak (around 100 years old) to sedimentary rocks in Utah (around 150 million years old).

Sedimentary rocks are often formed when sand or mud collects at the bottom of lakes and rivers and eventually compresses into rock. Sandstone can also form in a desert environment.* An excellent geologist I know taught me that in Arches National Park, the sedimentary rocks that later formed arches were formed in desert conditions similar to the dry heat of today.

An impressively big rock arch near Moab, UT

Even though sedimentary rocks can form in a dry environment, dinosaur tracks aren’t preserved if the footprints are on bare rock or in loose sand which blows away quickly. Fossils of dinosaur tracks tell us about the creatures who walked there and also a bit about the climate in that place and time.

Imagine, when all the mud and silt that formed these rocks settled all those years ago, this was a wet environment. Crocodiles slid through the mud and mats of algae grew in the shallow water (scientists know this because the crocodile tracks and algae texture formed fossils that we can see today).

When animals leave footprints in the sand, the prints blow away quickly, usually in less than a day.

Footprints in the sand of two different small animals and a human boot

But once in a great while, if an animal leaves a footprint in just the right kind of mud, the layer of rock forms in that shape. This is a three-toed fossil footprint from a dinosaur:

A dinosaur track (my hand for scale)

It’s just wild, to me, that we can see places where dinosaurs walked. Fossil bones are pretty cool too, but tracks—you can practically see the dino’s foot!

Here are some more dino tracks, if you look closely:

Dinosaur tracks

This dinosaur track site is on BLM land and was only found and reported in 2008. So relatively recently, paleontologists arrived and started identifying and interpreting these tracks. Several different kinds of dinosaurs and a crocodile walked around this mud flat, back in the day (112 million years ago in the early Cretaceous).

Much more recently, although still pretty long ago, the people who lived here etched petroglyphs into the darkened surfaces of sandstone cliffs, hundreds or a thousand years ago. These images include stylized humans, bighorn sheep, ravens, and other animals.

Petroglyphs

Visiting these sites respectfully seems straightforward: do your best to not damage these pieces of our history that have endured for so long already. At every one of these sites, recent human damage is obvious. Graffiti on or near the petroglyphs. Tracks worn away from people making molds or impressions or touching them. Fossil bones chipped out and removed from the original site. At the dinosaur tracksite, a news article from 2022 reported construction workers unintentionally driving over tracks while working on the boardwalk.

People certainly remove some fossils to study and place in museums, but I would hope it’s done fairly intentionally and not completely destructively.

Did you know that scientists now think many dinosaurs had feathers? Perhaps they looked more like giant birds than giant lizards. We don’t have many fossils of the skin or feathers of dinosaurs at all, so some of this is speculation. My (somewhat fantastical) mental image is of a giant raven, strutting intelligently like ravens do, working their way between the pine trees and palms in a muddy, wet salt flat. Kind of like Florida, but with giant creatures.

Somewhere in Florida with palm trees and water, last winter

The desert landscape here today preserves many traces of the people and creatures who lived here before. I’m grateful to see the stories pressed into and carved out of the rocks.

The sun sets over sagebrush, forming a gradient from orange to deep blue in the sky

*The initial version of this post said sedimentary rocks only form in a wet environment. This is untrue: sedimentary rocks (such as those in Arches NP) form in desert, dry, windy environments also. I still think dinosaur tracks are more likely to be preserved in mud than dry sand.

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Take care,

Rey