From North America’s largest known dinosaur tracksites at Picket Wire Canyonlands in southeast Colorado to Jurassic remains embedded in rocks at Dinosaur National Monument in western Colorado, it’s no secret the state holds many clues to a prehistoric past.
The separation in ages of reptiles and mammals happened about 66 million years ago, when an asteroid struck near Mexico’s Yucatán Peninsula, killing roughly 75% of the planet’s plant and animal life. The asteroid left a crater 93 miles wide and 12 miles deep, sending plumes of radioactive dust to the sky. The dust covered the sun, choking out the Earth’s prehistoric reptiles and plants, also resulting in volcanic eruptions.
As the dust settled over time, fossils deposited in the radioactive dust, forming their own layer in the Earth’s crust. This line, called the Cretaceous-Paleogene boundary was discovered in North America for the first time at a mountainous site in Colorado.
The Cretaceous-Paleogene boundary (K-Pg boundary), previously called the Cretaceous-Tertiary boundary (K-T boundary), is a representation of the separation between the age of reptiles and the age of mammals. The boundary, hidden within layers of rock, shows a dramatic difference in fossil deposits on either side of the line.
Also discovered in the K-Pg boundary is a “strikingly” high concentration of iridium – one of the rarest elements in the Earth’s crust, but a common metal found in meteorites, asteroids, and comets. The presence of iridium in the K-Pg boundary helped researchers finalize the theory of the asteroid strike as the cause for the mass extinction event.
The K-Pg Boundary was first discovered in North America in 1943 at South Table Mountain in Jefferson County. Hikers can reach this rare geologic feature from two trailheads on the southeast corner of South Table Mountain Park. There are about 4 miles of trails for hikers, mountain bikers, and horseback riders to use and see a 2.5-mile stretch of the K-Pg Boundary.
The Cretaceous Trail is a smooth, gravel road for about 1 mile, then hikers can take Basalt Cap Loop along the mesa’s flattop. The basalt rock in this area was formed from a volcano of the Front Range mountains about 64 million years ago – about 1.5 million years after the mass extinction event. Hikers and riders can continue from the Basalt Cap Loop and descend the Tertiary Trail to cross the K-Pg Boundary.
To spot the boundary within the rock, look for a gray clay-like material embedded in the basalt rock. Here you’ll see a glimpse into the prehistoric past of one of the most historic natural events the Earth has experienced.
Another location in Colorado where the K-Pg Boundary can be seen is Trinidad Lake State Park. This location of the boundary is one of the most exposed layers of the boundary and most easily accessible. Follow a quarter-mile trail to the interpretive signs that mark the K-Pg Boundary. Views can easily spot a layer of clay exposed in the sandstone rock.
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