PALMER LAKE, EL PASO COUNTY, COLORADO, GEOLOGY
by Darrel Dunn, Ph.D., PG, Geologist
Purpose and Scope
The purpose of this web page is to describe the geology in and near the town of Palmer Lake, Colorado. The description is general and non-technical. I avoid using technical terms that are not commonly understood and not in an internet accessible dictionary. Where it is helpful to use an obscure technical term, I define or discuss it.
Geologic Features in Vicinity of Palmer Lake
Figure 1 is a map showing geologic features in the vicinity of Palmer Lake. The map is based on detailed maps provided in references 1 and 2. However, Figure 1 is simplified by omitting detail primarily of interest to professional geologists. The most obvious feature in the area is the Rampart Range. The town of Palmer Lake sits at the base of this mountain range, which rises to elevations exceeding 9000 feet along its crest about five miles west of the town. The bedrock of the Rampart Range is granite. The mountainous granite terrane is terminated abruptly on its east side by the Rampart Range Fault (dashed line with triangles).
Figure 1. Generalized geologic map of Palmer Lake, Colorado and vicinity.
The bedrock east of the Rampart Range Fault is mostly sandstone and conglomerate composed of granitic material. The sandstone and conglomerate is layered. The layers result from differences in the size and composition of the grains, cementing material binding the grains together, clay content, and other internal features. There are some layers of claystone and siltstone. This sandy and clayey rock material is part of the geologic unit called Dawson Formation. The upper part of this formation is conspicuously exposed in cliffs around Ben Lomand Mountain and Elephant Rock. The area labeled Dawson Fm on Figure 1 is where the formation is at the ground surface or is covered by thin local deposits of colluvium or alluvium. Colluvium is loose material that has washed, rolled, and crept downslope due to gravity. Extensive deposits of colluvium around Ben Lomand Mountain are shown on Figure 1. Some smaller areas of colluvium are not shown. Alluvium present along small ephemeral streams in the Dawson Fm area is also not shown. Ben Lomand Mountain is capped by gravel, which is reported to be over 60 feet thick in places.
Monument Creek flows from the Rampart Range between Raspberry Mountain and Chautauqua Mountain and then flows southeastward parallel to Highway 105. It is joined by North Monument Creek, which exits the Rampart Range between Chautauqua Mountain and Sundance Mountain. These creek bottoms are filled with alluvium. A small gravel covered terrace parallels Monument Creek at Pine Crest. It is labeled Lower Terrace Gravel on Figure 1. It slopes upward away from the stream to about 40 feet above it at Shady Lane.
A prominent higher gravel covered terrace covers much of the area west of Highway 105. It is labeled Higher Terrace Gravel. It is divided into four sections in the area covered by Figure 1 - a section north of South Valley Road, a section north of Suncrest Road, a section crossed by Red Rock Ranch Drive, and a section crossed by Sunburst Drive. The sections are separated by eroded areas. This higher terrace slopes upward toward the Rampart Range from an elevation of about 7000 feet above sea level near Monument Creek to as high as 7400 feet near the mountain front. The gravel covers a bedrock terrace. The gravel on the terrace locally exceeds 15 feet in thickness, but the gravel has been eroded so that it may be thin locally and bedrock (Dawson Formation) may be exposed. The area around Douglas Avenue has been eroded to the extent that there is a mixture of Alluvium, Colluvium, and exposed Dawson Formation.
In the northern part of Figure 1, in the vicinity of Columbine Road, an Alluvial Fan slopes outward from the mountain front. Small streams flowing eastward from Sundance Mountain have deposited a conical apron of gravelly alluvium. The curving streets follow the level contours of the Alluvial Fan.
The mountain front is steep north and south of Monument Creek and loose material has washed, rolled, and crept downslope due to gravity, forming colluvial deposits. These colluvial deposits are around Largo Avenue and south of Suncrest Road. The colluvium has been eroded south of Rosita Avenue to expose Dawson Formation. An extensive area of Dawson Formation is present near the mountain front along red Rock Ranch Drive, Limestone Road, and Sandstone Drive.
The surfaces of all the geologic units have been modified by soil forming processes, except for cliffs and other surfaces too step to retain soil. The Dawson Formation areas are covered by loamy sandy soils. The Colluvium, Alluvial Fan, Terrace Gravel, and Alluvium areas contain gravelly, sandy, loamy soil. Soil depths on all of the geologic units may exceed five feet. Detailed descriptions of the soils may be found in reference 3.
GEOLOGIC HISTORY OF PALMER LAKE
A presentation of the geologic interpretation of the sequence of events that produced the geologic features on Figure 1 might add some perspective. The most relevant events date back more than a billion years. The duration of a billion years is hard to imagine. If a sheet of 24-pound paper were to represent a single year, a billion sheets would stack about 71 miles high - 47 times as high as Pikes Peak is above the Colorado Springs Airport.
The granite exposed in the Rampart Range is called the Pikes Peak Granite. It was formed over a billion years ago when molten rock material deep within the earth cooled and crystallized. During the next 500 million years, the rocks overlying the granite and the upper part of the granite itself were eroded away, and streams carried the material to the ocean, where it was deposited. The continental masses that existed at that time did not resemble today's continents. These ancestral continental masses floated around, converged, and split until the present configuration of continents began to appear about 100 million years ago. The continental masses were (and still are) floating on the Earth's mantle. They move due to very slow boiling of the mantle, which is a solid that can deform in a plastic manner. The mantle surrounds the Earth's core, and extends about a third of the way to the Earth's center. It is composed primarily of iron, magnesium, silicon, and oxygen. The Earth's core is a sphere extending from the base of the mantle to the center of the Earth. It is thought to be composed of dense iron and nickel alloy.
As the ancestral continents moved, they also rose and fell relative to sea level and were sometimes deformed due to compression and stretching to form mountains and seaways on their surfaces. Sand, silt, and clay carried to the seaways from the adjacent land formed layers of deposits. These deposits later turned to sandstone, siltstone, and shale due to pressure and chemical reactions when they were deeply buried beneath younger layers. Layers of limestone were formed from the shells of sea life. Such layers that were deposited between 500 and 65 million years ago are not exposed in the Palmer Lake area, but are present in the subsurface east of the Rampart Range Fault. Since successive layers were deposited on top of each other they become younger upward.
The Rocky Mountains began to form about 70 million years ago (paper stacked 5 miles high) when the rock layers of the mountain area were folded upward. This folding was due to some sort of compression of the Earth's crust in the region. In the Palmer Lake area, the eastern flank of the fold broke. That break is the Rampart Range Fault (Figure 1). The rocks on the west side of the fault slid up while the rocks on the east side moved down to form a basin (Denver Basin). As the west side moved up, it was eroded by streams that carried sediment into the Denver Basin as it subsided. The movement along the Rampart Range Fault was great enough to place the Pikes Peak Granite on the west side against much younger rocks on the east side. The rocks on the west side of the fault are estimated to have moved upward more than 12,000 feet relative to the rocks on the east side 1). This mountain building movement continued until about 40 million years ago.
The streams in the mountain area cut down into the Pikes Peak Granite, and the sediment deposited in the Denver Basin contains a lot of granitic material. Palmer Lake is on the very western edge of the Denver Basin, which extended over much of the eastern Colorado area at that time. This erosion and deposition continued, with some interruption, until a relatively flat land surface was produced by about 38 million years ago. This surface extended eastward across the Denver Basin area. In the Palmer Lake area the land surface was on granite west of the Rampart Range Fault and on the sediment derived from the granite east of the fault. The sediment derived from the granite and deposited in the Denver Basin formed thick deposits that were lithified due to the high temperature and pressure of deep burial. Some of these deposits became the Dawson Formation that we see in the Palmer Lake area exposed on the cliffs of Ben Lomand Mountain and Elephant Rock. The Dawson Formation is also exposed in rock pillars and small cliffs near the mountain front.
The erosion surface produced 38 million years ago was not completely flat. Some uneven topography existed in the mountain area, and the Pikes Peak portion was higher than the surrounding land. Subsequent to the development of the erosion surface, there was volcanic activity in the area that is now the Rocky Mountains, and thick deposits of incandescent ash were spread over a wide area. This ash cooled to form a layer of dense volcanic rock, which has been dated at 36.7 million years ago. Subsequent to the volcanism, stream erosion was intensified due to regional uplift and wet climatic conditions. The volcanic rock and other bedrock were eroded and course gravel containing fragments of the volcanic rock was deposited in valleys that extended across the Denver Basin. Such gravel now caps Ben Lomand Mountain (Figure 1). The gravel was deposited about 36 million years ago.
Most of the gravel deposited at that time has been removed by subsequent down-cutting of streams. This down-cutting has continued to the present day, producing the Arkansas River and Platte River drainage basins separated by the Palmer Divide. The erosion of the relatively soft Dawson Formation has been more rapid than the erosion of the hard granite west of the Rampart Range Fault. Thus the Rampart Range has been left high relative to the area east of the fault, where bedrock is Dawson Formation. There have been pauses in the erosional down-cutting due to interruption in the regional uplift and/or changes in climate. During these pauses, the streams wondered laterally and eroded bedrock terraces that became covered by gravel during the late stage of each pause. Several such surfaces were formed in Colorado. Two of them are partially preserved in the Palmer lake area - the Higher Terrace Gravel and the Lower Terrace Gravel shown on Figure 1. The terrace represented by the Higher Terrace Gravel was probably formed about 640,000 years ago (paper stacked 240 feet high) between the second and third stages of continental glaciation. The terrace represented by the Lower Terrace Gravel was probably formed between 100,000 and 250,000 years ago between the third and fourth stages of continental glaciation. The alluvium along the present streams has been deposited during down-cutting after the fourth (most recent) stage of continental glaciation. The continental glaciers (nor the associated mountain glaciers) did not reach the Palmer Lake area, although glacial deposits have been found at high elevations in the Pikes Peak area. However, erosion in the area was affected by the glacial and interglacial climate change. We are currently in a period of climate warming, retreat of most glaciers, and sea level rise associated with the waning glaciers.
Close to the mountain front, colluvium and alluvium have washed down over the Higher Terrace Gravel (Figure 1). Colluvium has also formed around Ben Lomand Mountain. Most of this deposition of colluvium and alluvium probably occurred during wet climate associated with continental glaciation. All of the colluvium, alluvium, and bedrock have been altered by soil forming processes to produce the land surface we see today.
In summary, features shown on Figure 1 reflect major events in the geologic history of the area - igneous intrusion of the Pikes Peak Granite, mountain building forming the Rampart Range Fault, basin filling forming the Dawson Formation, and removal of part of the Dawson Formation by erosion. This erosion left the Rampart Range standing high, produced the gravel covered terraces, and the present stream valleys. Some of these features have been covered by material washing and sliding down the slopes. Soil has formed on the geologic materials exposed at the land surface.