D2 Synorogenic Strata:
This layer contains several hundred feet of coarse gravel and sandstone layers derived from the uplifted Rocky
Mountains. Environments resembled debris fans east of the modern Andes Mountains in South America. These sediments
were deposited in the Eocene Epoch (55-50 m.y.a). Typical fossils include petrified wood, leaves, and rare mammal
teeth.
Dawson Arkose:
A sedimentary unit rich in quartz and feldspar typified by outcrops on Dawson Butte and road cuts on Interstate
25 near Larkspur. The Dawson Arkose intertwines with the Denver Formation, making regional mapping difficult. Our
terminology ascribes the lower portion of the Dawson Arkose to D1 Synorogenic Strata and the upper portion of it
to D2 Synorogenic Strata.
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D1 Synorogenic Strata:
This layer is composed of approximately 1,500 feet of alternating layers of sandstone from ancient river channels
and mudstone from ancient flood plains. These layers were deposited during the early part of the uplift of the
Front Range by rivers similar to the modern Platte River in northeastern Colorado. D1 Synorogenic Strata are coarse-grained
along the Front Range and become fine-grained and coal-like to the east. These sediments were deposited in the
Cretaceous Period and Paleocene Epoch (68-56 m.y.a). Typical fossils include petrified wood, leaves, dinosaur bones,
and mammal teeth.
Paleosol:
A brilliant red clay layer up to 60 feet thick lies on top of the D1 Synorogenic Strata. This red clay formed as
ancient soil, similar to modern red soils that have formed in the moist climates of the southeastern United States
and the Amazon Basin of Brazil. The presence of the layer suggests that it was exposed and weathered for a very
long period of time, perhaps as long as 9 million years. The clay is mined to make bricks.
Denver Formation (Paleocene part):
A sedimentary unit rich in volcanic debris typified by exposures on South Table Mountain west of Denver. Our terminology
ascribes the Denver Formation to D1 Synorogenic Strata. Typical fossils include palm and sycamore leaves and mammal
teeth. The Castle Rock Rainforest site is located at the top of this unit, and the Denver International Airport
fossil site is near its base.
Denver Formation (Cretaceous part):
A sedimentary unit rich in volcanic debris typified by exposures on South Table Mountain west of Denver. Our terminology
considers the Denver Formation to be part of the D1 Synorogenic Strata. Typical fossils include palm leaves and
dinosaur bones. The Colorado Rockies mascot, Dinger the Dinosaur, was named because home plate at Coors Field is
located in the dinosaur-bearing portion of the Denver Formation.
Arapahoe Conglomerate:
A thin layer of gravel, representing the early debris eroded from the uplifting Rocky Mountains. Commonly contains
fragments of granite and metamorphic rocks, indicating the streams were eroding the bedrock of the uplifted mountains.
Our terminology considers the Arapahoe Formation to be the basal portion of the D1 Synorogenic Strata. Typical
fossils include palm leaves and dinosaur bones.
K-T Boundary:
The boundary between the Cretaceous (age of dinosaurs) and the Tertiary (age of mammals) Periods. This interval
is often characterized by a half-inch-thick layer of debris that is the fallout from a massive comet or asteroid
impact event in Mexico. The impact eject covered Earth with a mantle of dust and probably caused the extinction
of the dinosaurs and their world.
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Laramie Formation:
This formation was deposited by low-lying coastal swamps and estuaries along the edge of the Cretaceous sea. Coal
seams are common in the lower part of the formation. Typical fossils include palm leaves and dinosaur bones.
Fox Hills Sandstone:
These deposits accumulated at the margin of the retreating Cretaceous Western Interior Seaway, the remains of ancient
beaches and shallow marine sand layers. Typical fossils include clams and the burrows of shoreline shrimp.
Pierre Shale:
This mile-thick layer of dark gray mudstone and shale accumulated at the bottom of the Cretaceous Western Interior
Seaway. Clams, ammonites, and baculites are the most common fossils. Their distribution tells that for millions
of years, this seaway connected the Gulf of Mexico to the Arctic Ocean and covered most of what today is Colorado.
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