Colorado geologic history shows the Pikes Peak area was covered by a vast, shallow sea. The sediments accumulated until they reached 40,000 feet. With time, chemical concretion, and pressure, they became sedimentary rock. Then during a period of orogeny (mountain-building), metamorphic rocks were formed. These rocks are called the Idaho Springs Formation and are located nearby in Idaho Springs, Colorado.
One billion years ago, in another episode of orogeny, a huge batholith (a gigantic igneous rock) was forced upward from deep below the Earth’s surface. This batholith is what is referred to locally as the Pikes Peak batholith and the rock is Pikes Peak granite. It is now exposed over an area of hundreds of square miles.
Over time, the ancestral Rockies began to erode and finally became a flat, slightly undulating terrain called a peneplane.
From 600—500 million years ago, the Pikes Peak granite and the Idaho Springs metamorphics were formed. The sea gradually transgressed from both the west and the southeast, and perhaps 530 million years ago, the seas met in the Colorado Sag. The beaches transgressed, thus forming the Sawatch Sandstone. The sandstone underwent a gradual transition into the red and greenish carbonate rock of the Peerless Formation. These were deposited after the sea transgressed and formed either limestone formations or dolomite.
About 425—500 million years ago, gentle uplift and erosion removed a large amount of the Peerless Formation in this area. The sea deepened again, and a period of intense carbonate deposition began. The Manitou Formation was deposited, consisting of nearly 200 feet of dolomite and limestone. Today, this formation is red or buff-gray. Fossils, such as trilobites, can be found in the Manitou and remaining Peerless Formations. During this period, the sea retreated, but then again transgressed. This resulted in the sandstone and shale Harding Formation and the carbonates of the Fremont Limestone. Unfortunately, both of these formations were almost entirely eroded locally, but can be seen to the south near Cañon City, Colorado.
According to some geologists, there was deposition in the entire Rocky Mountain area from 345—405 million years ago. There is evidence of a shallow inland sea. However, erosion may be responsible for the lack of carbonates of this period with one significant exception: the Williams Canyon Limestone was formed. All of these deposits contained layers of limestone formations and dolomite, as well as thin layers of sandstone and shale.
Between 405—425 million years ago, most of the “beach” sandstone (Harding Formation) and the carbonates (Fremont Limestone) eroded away.
From 310—345 million years ago, there was a brief period of non-deposition before the land gently subsided and the sea once more covered this area. The deposition of the Williams Canyon Limestone and the Hardscrabble and Beulah Formation were from this time, and all of these deposits contained layers of both limestone and dolomite, as well as thin layers of sandstone and shale. Later, when the land rose again, erosion caused much of the Hardscrabble and Beulah Formation to erode away. Another name for the remaining Hardscrabble Formation is Madison Limestone. All of the depositions in this geologic era have been grouped together and are known as the Leadville Limestone. The most significant development (for the Cave of the Winds) was the formation of our amazing Colorado caverns.
In a fairly humid climate with abundant rainfall and carbonate surface rock, the rainwater dissolved the rock and formed many sinkholes, caverns, and tunnels in the land. This type of landscape is called a karst topography. The many caves in Williams Canyon are more recent developments.
From 230—280 million years ago, the Ancestral Rockies were considerably worn down and formed a peneplane. A Sahara-like arid climate prevailed and dune sands developed in the region. These dunes developed into a sandstone formation known as the Lyons Sandstone. These quite clearly indicate the cross-bedding typical of windblown sand. This rusty-colored sandstone is in the Garden of the Gods. Toward then end of this time, the eastern sea again returned and the sand, shales and thin limestone layers of the Lykin Formation were deposited.
The eastern seas retreated from the Pikes Peak area in time period from 135—180 million years ago. No evidence of any more deposition for the next 50 million years is apparent. The local landscape consisted of swampy lowlands and meandering streams connecting numerous fresh water lakes. Clay, silts, shales, and carbonates accumulated the most extensive and famous formation of the Rocky Mountain area—the Morrison Formation. Many dinosaurs lived in this area and the fossil remains of the Stegosaurus came from the shales of the Morrison Formation north of Cañon City. In the Garden of the Gods, a layer of gypsum is located at the base of the Morrison Formation.
The swamp lowland condition continued into the early period spanning 70—135 million years ago. The Purgatoire Formation (seen on Gold Camp Road near Section 16) shows a gradual transition from the lowland conditions to a marine environment, when the great Cretaceous seas covered this area during most of the remainder of the period. The Purgatoire Formation consists of two members: the lower Lytle Sandstone and the upper Glen Cairn Shale. The Lytle Sandstone is nearly white, contains very coarse grains and is sometimes considered a conglomerate. With the accelerated subsidence of the land, beach sands accumulated along the shore of the sea that advanced from the northeast. These sands formed the buff-colored Dakota Sandstone. This formation is rich in preserved ripple marks and iron-stone concretions. These concretions are round little balls of darker brick, colored by iron oxide.
The Purgatoire and the Dakota Formations are referred to as the Dakota Group. As the shallow seas advanced to cover the beaches, the deposition changed from sand to shale to limestone. The resulting formations form the Benton Group. They comprise approximately 500 feet of strata and include Graneros Shale, Greenhorn Limestone, and Carlisle Shale.
As the land subsided once more and the sea deepened, a very thick layer of dense, gray limestone, called the Fort Hays Limestone, was left.
The next deposition in this sea (Pierre Seaway) was the Pierre Shale. This is the thickest of all the local sedimentary strata; approximately 5,000 feet of coarse gray limestone. This formation is very rich in fossils, including baculites, clams, mosasaurs, extinct crabs, horsetail ferns, cycads, and other plants.
Near the end of this time, the last of the great inland seas retreated to the northeast. The receding shoreline resulted in the Fox Hills Sandstone, which is 200—300 feet in thickness. Also at this time, the lagoons and swamps provided the dense vegetation that later became the coal beds of the Laramie Formation. To the north of Colorado Springs is the Denver Formation, a sandstone-shale formation resulting from the retreating sea.
From 1—63 million years ago, the Laramide Orogeny was experienced locally as gentle uplift that caused the retreat of the Cretaceous Sea (Pierre Seaway). More intense uplift gradually progressed, resulting in a huge block of crustal material thrust upward 10,000 to 15,000 feet by a series of uplifts. Great faults occurred where the blocks broke away from the surrounding rock, and the ancient Pikes Peak Granite was lifted into contact with Mesozoic and tertiary sedimentary strata. The fault that begins in the Garden of the Gods and runs north for about 30 miles is the Rampart Range Fault. The fault that runs south of Cheyenne Mountain westward through Ute Pass is the Ute Pass Fault. The Ute Pass Fault is the result of a huge piece of the exposed Pikes Peak Batholith being thrust upward and outward over the sedimentary strata to the east.
In the northern part of the area, and under much of the Air Force Academy and Black Forest, is a stratum called Dawson Arkose, which is coarse sandstone and conglomerate deposited during the erosion of the rapidly rising Front Range. A number of volcanoes were known to have erupted in this area during the Laramide Orogeny. The Cripple Creek gold mines are located in the caldera of an extinct volcano, as well as the 39-Mile Volcanics. The Florissant Fossil Beds were formed when volcanic ash covered a fresh water lake in that area, preserving many plants and insects. The Castle Rock Conglomerate was formed in this period also.
Over the last million years, the local area was too far south for continental glaciation, but in the Rocky Mountains, alpine-type glaciers grew and spread down to an elevation of approximately 9,500 feet. The ice carved bowls (called cirques) out of the mountain faces. When the ice moved down the valleys, great quantities were carried along the glacier, forming lateral and terminal moraines. The melted water carried enormous amounts of material from the mountain toward the plains. This material became unconsolidated and graded deposits known as quaternary alluvium. The Mesa gravels, which form the flat topped hills around Colorado Springs, represent the outwash from melting glaciers.
Geology Links of Interest
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