Rocks and Dirt
- Tom Shankapotomous
- Oct 1
- 4 min read
Why Think About Geology?
When you spend a lot of time on trails, it’s natural to start wondering about the ground under your feet. It’s not just dirt and rocks—it’s Earth’s history laid out before you, shaped by millions of years of volcanic eruptions, shifting continents, and relentless erosion. Geology is the Earth’s autobiography, and "deep time" is the timescale it’s written on: roughly 4.6 billion years of change. I’m not a geologist by training, but I’ve grown more curious about the geologic stories behind the places where I run. What follows is a mix of what I’ve read, thought about, and tried to make sense of.
So, What Do Geologists Actually Do?
Geologists study Earth—its materials, its history, and the forces that shape it. They’re the folks who figure out how mountains form, why earthquakes happen, or what long-lost environments looked like based on the rocks left behind. They also help with real-world problems, from finding water and resources to understanding natural hazards and climate change.
The science really started to take shape in the late 1700s. James Hutton introduced the idea of "deep time," arguing that Earth’s changes happen slowly over eons. Later, Charles Lyell helped popularize that thinking, and over time, tools like radiometric dating and satellite imagery have turned geology into a full-on detective story of the planet.
I. Geology in a Nutshell
Geology is about understanding what the Earth is made of, how it got that way, and how it keeps changing. It covers how rocks form, how mountains rise and crumble, and how rivers and glaciers carve the land. Geology also helps us make sense of climate change, natural disasters, and even where to build safe roads or find drinking water.
Some Key Questions Geologists Ask:
• What is this stuff? (Minerals, rocks, fossils—what are they and how did they form?)
• How is it arranged? (Layers, faults, folds, intrusions.)
• When did it happen? (Relative and absolute dating of events.)
• What happened here? (Old rivers? Volcanoes? Glaciers?)
• Why does it matter now? (Hazards, resources, ecosystems.)
The Rock Cycle and Time:
• Rock cycle: Igneous rocks form from cooled lava or magma. Weathering breaks them down into sediments, which become sedimentary rocks. Pressure and heat can turn any rock into metamorphic rock. These rocks can then be melted, uplifted, or eroded, keeping the cycle going.
• Deep time: Geologists read Earth’s history using clues like rock layers, fossils, and radiometric dating—like reading chapters in a very old, very long book.
How Geologists Work in the Field:
Fieldwork is hands-on. It means hiking around with tools, maps, and notebooks, observing rocks directly to figure out what happened in the past. Here’s a taste of what that looks like:
• Get the lay of the land: Study satellite images, past maps, local geology.
• Check out the rocks: Use hand lenses, test hardness, look for fossils or layering.
• Measure things: Use a compass to record angles and directions of rock layers and faults.
• Piece it all together: Build a story from the layers, clues, and structures you find.
• Map it: Draw geologic maps and cross-sections to show how things fit below the surface.
• Take samples: Bring back rock samples for lab tests or dating.
• Stay sharp: Take detailed notes, photos, and always question your assumptions.
Why It Matters:
Good fieldwork grounds all the other sciences. It turns scattered observations into a timeline of what happened and helps scientists understand everything from earthquakes to water flow.
II. Deep Time: Earth’s Massive Timeline
Earth’s history is broken up into chunks: eons, eras, periods, and epochs. These divisions are based on big shifts in climate, life, and geology.
The Four Eons (from oldest to now):
Hadean (4.6–4.0 billion years ago): Earth forms, crust starts to cool, first oceans and atmosphere appear.
Archean (4.0–2.5 Ga): Continental crust forms, early life (mostly microbes) shows up.
Proterozoic (2.5 Ga–541 Ma): Oxygen builds up, more complex life begins.
Phanerozoic (541 Ma–present): Explosion of visible life and most of the fossils we know today.
The Phanerozoic in Three Eras:
Paleozoic (541–252 Ma): First fish, land plants, and forests. Ends with the biggest extinction in history.
Mesozoic (252–66 Ma): Age of dinosaurs, breakup of Pangea, ends with the asteroid that wiped out the dinos.
Cenozoic (66 Ma–today): Age of mammals, ice ages, and continents reaching today’s positions.
Recent Highlights:
Quaternary Period (last 2.6 million years): Glacial cycles shaped many of the landscapes we run through today.
Holocene Epoch (last ~11,700 years): Basically everything since the last ice age.
Anthropocene? Some scientists argue we’re now in a new epoch shaped by human activity, but it’s still being debated.
Why This Matters to Trail Runners
Once you start noticing the geology beneath your feet, the trail changes. Sandstone might give you solid grip one day and turn slick when it’s wet. Shale can crumble into ankle-twisting scree. Granite makes for fun scrambling, but you have to pay attention. Even the grade of a trail often follows the tilt of the rock layers or cuts across old fault lines.
Geology also explains why the terrain looks the way it does. Glaciers left behind big boulders and U-shaped valleys. Ancient seas laid down limestone beds. Volcanic activity built up high ridgelines and left behind pumice fields.
Understanding this deep history can shift your mindset. Trails aren’t just routes—they’re the outcome of forces that have been shaping the Earth for millions of years. Roads are designed to flatten and standardize, but trails reveal. They let you experience the raw, unsmoothed character of the land. Once you learn to see the rocks and forms for what they are, every run becomes a kind of time travel.
References
National Park Service. (n.d.). Geologic time overview. U.S. Department of the Interior. https://www.nps.gov/subjects/geology/time.htm
Geology.com. (n.d.). Geologic time scale. https://geology.com/time.htm
Gradstein, F. M., Ogg, J. G., Schmitz, M. D., & Ogg, G. M. (2020). Geologic time scale 2020. Elsevier.
Macdougall, D. (2011). Why geology matters: Decoding the past, anticipating the future. University of California Press.
Monroe, J. S., & Wicander, R. (2020). The changing Earth: Exploring geology and evolution (8th ed.). Cengage Learning.
Press, F., & Siever, R. (2001). Understanding Earth (4th ed.). W.H. Freeman and Company.
U.S. Geological Survey. (n.d.). Geologic time. https://pubs.usgs.gov/gip/geotime/
Zalasiewicz, J. (2008). The Earth after us: What legacy will humans leave in the rocks? Oxford University Press.
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