Western Kansas holds a special place in my heart. My grandparents lived in Tribune, Kansas, so growing up, we spent many summers driving north from Oklahoma across the open plains.
Every so often, something extraordinary would come up on the horizon—massive white rock towers emerging from the prairie like ancient ruins from another world.
Monument Rocks and Little Jerusalem Badlands State Park tell an incredible geological story, representing one of the best windows into the ancient Western Interior Seaway. If you’re road-tripping through western Kansas, they’re well worth the detour.
Molly Turko
Structural Geologist, Devon Energy
Monument Rocks
Monument Rocks at Sunset/ Photography by Bob Craig/Shutterstock
Monument Rocks is located south of I-70 near Oakley, Kansas. Sunrise and sunset provide spectacular lighting for photography. Here's a look at the site's must-see geology.
Area geology: Monument Rocks is composed of the Upper Cretaceous Niobrara Chalk Formation, deposited roughly 85 million years ago when western Kansas was submerged beneath the Western Interior Seaway—a vast inland sea that once split North America in two.
The chalk formed from the accumulation of microscopic calcareous plankton, primarily coccolithophores, that settled to the seafloor over millions of years.
These sediments were deposited in a quiet offshore marine environment below normal wave base, where fine carbonate mud accumulated in oxygen-poor waters.
Wind, rain, and erosion later sculpted the soft chalk into the towering arches, pinnacles, and buttes seen today.
The Niobrara and associated Upper Cretaceous marine shales are important regional source rocks within the Western Interior petroleum system, helping charge many Rocky Mountain basins.
Fossils finds:
Monument Rocks and the Niobrara Chalk are famous for marine fossils from the ancient seaway, including mosasaurs, plesiosaurs, sharks, ammonites, giant inoceramid clams, and predatory fish such as Xiphactinus.
One of the most famous Niobrara fossils is the “fish-within-a-fish” specimen, where a 14-foot-long Xiphactinus was preserved with another fish still inside its stomach.
Fun fact: Monument Rocks was designated a National Natural Landmark in 1968, with some chalk towers rising more than 70 feet above the plains.
Trip tips: Continue west to Little Jerusalem Badlands State Park for even larger exposures of the Niobrara Chalk.
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Little Jerusalem Badlands State Park/ Photography by David Radzieta/Shutterstock.com
Unlike the isolated towers at Monument Rocks, Little Jerusalem Badlands State Park exposes sweeping chalk canyons and badlands topography that feel like stepping directly onto the floor of the ancient Western Interior Seaway.
The park officially opened to the public in 2019 and is located about 20 minutes west of Monument Rocks.
Area geology: Little Jerusalem exposes the Smoky Hill Chalk Member of the Upper Cretaceous Niobrara Formation.
Thin marl and shale beds reflect subtle changes in sea level and sediment input.
The dramatic badlands formed through ongoing wind and water erosion of the soft chalk across the semi-arid plains.
Fun fact: Little Jerusalem contains the largest exposed Niobrara Chalk outcrop in Kansas.
Fossil finds: The Smoky Hill Chalk is internationally known for exceptionally preserved marine fossils, including mosasaurs, sharks, turtles, fish, ammonites, and even flying reptiles such as Pteranodon.
Trip tips:
Stay on designated trails to protect fragile fossils and outcrops.
The Overlook Trail provides some of the best views in the park.
Spring and fall offer the most comfortable hiking conditions.
Go deeper: Learn more about the geology of western Kansas here.
