Roxborough State Park is a designated Colorado Natural Area, National Cultural District, National Natural Landmark and a Leave No Trace Gold Standard Designated Site.
The most striking feature for visitors to Roxborough is the dramatic Fountain Formation. This spectacular tilted sandstone began over 300 million years ago with the gradual erosion of the Ancestral Rocky Mountains. Today these red sandstones stand beautifully at Roxborough at a sixty degree angle and are the result of millions of years of uplift and erosion.
There are excellent examples of exposed geology from the Precambrian to Late Mesozoic, including hogbacks of Cretaceous, Permian, and Pennsylvanian age. Erosion of steeply dipping monoclinal sedimentary sections has resulted in the series of three major hogbacks and strike valleys, exposing highly scenic dipping plates, spires and monoliths. Precambrian gneiss and biotite-muscovite granite are exposed on Carpenter Peak.
Geology of Roxborough State Park
Here at Roxborough State Park the sedimentary rocks of the Great Plains transition with the crystalline basement rock of the Rocky Mountains.
As a result, billions of years of Front Range geology is exposed in spectacular ways:
- The red spires and monoliths of the Fountain formation are evidence of a mountain range older than the Rockies
- The Dakota Hogback marks the western edge of an ancient inland sea
- The granite and gneiss of Carpenter Peak tell us of the great forces of heat and pressure deep inside the Earth.
This grand display of Font Range geology, along with its biological diversity, lead to Roxborough's designation as a National Natural Landmark and a Colorado State Natural Area. At Roxborough State Park you can see part of the story of the ever changing landscape of Colorado and the Rocky Mountain Region.
Geologic History of Roxborough
The geologic story of Roxborough begins nearly two billion
when ancient sediments were buried at great depths within the earth.
Here, they were subjected to immense heat and pressure due to the collision of
ancient crustal plates, turning them into metamorphic rock. Millions of years
later, magma from deep within the Earth moved up through cracks in the
metamorphic rock and created a massive chamber that cooled over millions of
years into solid granite.
There is a mysterious period between the cooling of the
granite and the first sedimentary rocks seen in the park, as nearly 1 billion
years of geologic time is missing between these two layers. This is likely due to
a long period of erosion. Large breaks in the rock record like this are
referred to by geologists as unconformities. Because this one represents such a
large break in time it is typically referred to as The Great Unconformity.
Around 300 million years ago...
a mountain range formed in
present day Colorado. This range is known as the Ancestral Rocky Mountains, and
is in fact a completely different range from the Rockies we know and love
today. As these mountains rose, they began to erode into large sheets of
sediment, producing the oldest sedimentary rocks seen in the park. The
mountains continued to erode until the region became relatively flat. This
ancient flat surface still remains on top of some fourteeners today.
Several sedimentary layers were deposited in the relatively
flat Colorado Region from roughly 300 to 65 million years ago. Each layer was a
product of different climates and environments including: a desert, tidal flat,
floodplains and swamps, beach, and even a shallow sea. These layers were later buried
where chemical and physical forces turned the loose sediment into solid rock.
Around 65 million years ago...
The shallow sea began to
retreat and a new mountain building event began. Although its exact cause is still debated
among scientists, it is believed that the collision of tectonic plates on the
west coast of North America was a major influence. As these new mountains rose
up they folded the overlying sedimentary rock, tilting it from its original
horizontal position. In Roxborough, the
rocks were tilted as much as 60 degrees. Over millions of years, erosion from wind,
rain and ice began to sculpt the landscape. Soft rock eroded quickly forming valleys,
while harder rock eroded slowly forming ridges.
As the Rockies grew, they began to shed lots of sediment
into the newly formed Denver Basin to the east. This produced many formations
including, the Denver, Arapaho, Dawson, Green Mountain Conglomerate, Castle
Rock Rhyolite, and Castle Rock Conglomerate. Although these rocks are not
present in Roxborough, they can be found east of the park and under most of the
Denver Metro area.
Today there are three main hogbacks in Roxborough:
Drainages such as Willow Creek have formed in the softer rock between these ridges and occasionally cut through them. The oldest Precambrian rock now lies on the west side of the park where Carpenter Peak, the highest point in the park, resides. The rock layers gradually get younger as you move east, out toward the plains.
The geologic history of Roxborough is one of continual change. At least two mountain building events, changes in climate and sea level, and the traces of ancient life are all recorded in the rocks of Roxborough State Park.
Geologic Formations at Roxborough
Below is a brief description of the major rock layers, also called formations, of Roxborough State Park, from oldest to youngest (west to east).
- Precambrian Basement Rocks (Older than 542 Ma)
The oldest rocks in the park are the Precambrian crystalline rocks of the Idaho Springs Gneiss (pronounced "nice") and Pikes Peak Granite, 1.7 and 1.2 billion years old respectively. The Idaho Springs Gneiss is composed of many layers of metasediments - sedimentary rocks that were transformed through heat and pressure into metamorphic rocks in the oldest recorded collision of tectonic plates in the Roxborough area. The Pikes Peak Granite on the other hand formed when molten magma intruded into the Idaho Springs Gneiss, later cooling into the rocks we see today. The distinctly pink color of Pikes Peak Granite is due to the mineral microcline, a form of feldspar. These rocks can be seen on Carpenter Peak, the highest point in the park at 7,160 ft in elevation.
- Fountain Formation (300 Ma)
The Fountain Formation is the most prominent geologic feature of the park with its bright red spires and monoliths. This coarse-grained sandstone is composed of sediments eroded from the Precambrian rocks mentioned above. If you look closely you can even see small pieces of the pink feldspar from the Pikes Peak Granite in the Fountain Formation. This sediment was eroded from the Ancestral Rocky Mountains (a range much older than our current Rockies) and deposited by sand-choked rivers into great fan shaped deposits called alluvial fans. In addition to the pink-colored feldspar grains, the Fountain formation gets its brilliant red color from an iron-rich clay mineral, called hematite, which holds the sand grains together. The west side of Fountain Valley Trail provides an excellent view of this formation.
The Lyons formation is also sandstone, but this one is noticeably lighter in color. This is because it was deposited as sand dunes in an ancient desert. As the Pangaea Supercontinent formed, the center of the continent became drier as it was cut off from major bodies of water, forming the vast inland desert that held these dunes. Rivers flowing from the low hills that were once the Ancestral Rockies carried sediment out into this desert, where wind reworked the sand again and again, until all that was left was "clean" sand composed almost entirely of quartz. Lyons Overlook and South Rim Trial give you an up close look at this formation and the cross bedding that is so indicative of sand dunes.
- Lykins Formation (250 Ma)
Just east of the Lyons is the relatively thin Lykins formation. This limestone and red clay layer was deposited in a harsh tidal flat where only mats of algae could grow. The sticky algae mats trapped sediment as they grew forming thin banded layers in the Lykins. This can be seen in some of the stones used to build the Persse Place. While this layer was being deposited, the greatest mass extinction in Earth's history occurred, known as the Permian-Triassic Catastrophe. It is estimated that as high as 90% of marine species and perhaps almost just as many land species became extinct during this time frame. This could account for why there are so little fossils other than the algae found in this formation.
This soft sandstone and shale layer was deposited during the Jurassic Period, a time dominated by dinosaurs. Many dinosaur fossils have been found in this formation throughout Colorado including near Morison, CO, the formation's namesake. No fossils have been found within Roxborough State Park, but any visitor that has hiked South Rim after rain is very familiar with the sticky, clay rich mud it produces. This clay comes from the environment in which the Morrison was deposited: a vast network of floodplains and swamps that covered much of present day Colorado throughout the Jurassic Period. The Morison Formation also shows how vegetation can indicate a change in rock type as it forms the grassy valley, between the shrubby ridges of the Lyons and Dakota Hogbacks.
- Dakota Group or Dakota Hogback (100 Ma)
The Dakota Group, or Dakota Hogback as it's commonly known, is a group of many small formations that for simplicity are combined into one large layer. It is primarily composed of beach and near shore sediments laid down on the west shoreline of a seaway that began to flood the center of the North American Continent around 100 million years ago. The soft sand of this beach preserved ripple marks from waves crashing against the ancient beach, visible from the entrance road to the north (your right) just before you pass through the Dakota road cut. The Dakota Group forms the east most hogback in the park and is best observed from the Dakota Ridge Overlook.
As the shoreline progressed westward, Colorado became inundated by a shallow sea. Rivers from distant mountains brought large amounts of clay and silt to this sea where it was deposited. This is how the Pierre Shale formed. Scientists have estimated that the depth of the sea reached around 600 ft at its maximum depth. This provided a great habitat for many sea creatures of the Cretaceous. Fossils of these creatures have been found throughout this formation including ammonites, shark teeth, and pterosaurs (flying reptiles). Check out the Cretaceous Sea graphic and rock that includes some of these fossils inside the Roxborough State Park Visitor Center. In Roxborough the Pierre Shale underlies the plains east of the Dakota Hogback.