Continuing our course southward, we find some curious mesas in the valley of West Plum Creek. We ascended one lofty butte, with a flat table summit, situated west of Plum Creek road. The top of this butte is about one thousand feet above the road, and is capped with a rather close-grained, cream-colored rock, which looks quite porphyritic, fifty to one hundred feet thick, and plainly of igneous origin.
Annual Report for the Years 1867, 1868, 1869, 1873 (Hayden 1873, 140)
This view drawn from the top of Raspberry Butte just southwest of the small town of Larkspur is appropriately entitled “Pleasant Park from the South.” The pleasing character to this landscape is a juxtaposition of bucolic ranching country, as if right out of an old western movie, and a geologic theme park that could have been produced as a diorama in a natural history museum. The geologic structure lies below the artist like an open book and is much more dramatic and aesthetically enjoyable in the drawing with its lack of vegetation than in the modern photograph. But the intricate geologic landscapes in the foreground that are backed by the seemingly less complex high mountains are real and tell an interesting and nearly complete geologic story that is millions of years old.
Unfortunately, the name Pleasant Park is no longer used—it was such a descriptive and apropos term for this scene. In fact, the name had already been changed to Perry Park when William Henry Holmes drew this landscape in 1874, but Holmes and the survey continued to use the old nomenclature. The land seen here was purchased by John D. Perry in 1872 and developed into a large cattle ranch. While the survey studied the land, Perry was in the process of imprinting his own stamp on the official deed. The name change, however, was slow in being universally adopted, probably because the Pleasant Park moniker was much more pleasing to the ear.
The geology depicted in this drawing could be used as a real-life text on the stratigraphic history of the Front Range of Colorado and the piedmont terranes of the sedimentary rock suite lying just to the east of the mountain front. The highlights of the geologic landform story in this view begin in the interval of time between the mountain-building episode that formed the Ancestral Rockies some 300 million years ago and the more recent Laramide orogeny that established the current pattern of the mountains of the Front Range. During the interlude between orogenies, lasting 235 million years, there was an often-repeated sequence of shallow seas moving into and out of this part of Colorado. With each successive transgression and regression of the water, a sequence of open if shallow water, lagoon formation, coastal dunes, and low-lying terrestrial environments developed in turn along the fluctuating littoral zones.
We see many of the sedimentary formations in this landscape repeated in other sections of this website (see the panoramic view in Colorado Springs/The Pikes Peak Group and the Section at Bear Creek/Morrison sections, for example). Small outcroppings of the famous red rocks of the Fountain formation lie close to the mountain front at the far left side of the scene. This rock consists of the erosional remnants from the rapid uplift of the Ancestral Rockies. After that major orogeny, the land of what we now call eastern Colorado was at much, much lower elevations, and this sea/estuary/marshland environment prevailed for eons. The next younger sedimentary formation after the Fountain is the Lyons, which derived from a combination of low lagoonal environments and coastal sand dunes. The sandstone that developed from the Lyons’ formation dunes was a relatively resistant rock and forms one of the major hogbacks at the left side of the drawing. The valley lying just east-northeast of the Lyons is underlain by the much more erodible Lykins formation—a shallow water, tidal-zone rock with large, fossilized algae blooms frozen in time that are visible today. These ancient algal features are formally known as stromatolites. Low-lying outcrops of the Jurassic age Morrison formation are also found in the valleys as we move our view northeastward. The Morrison is world famous for containing dozens of species of fossilized dinosaur remains in incredible density (see again section 5, on Bear Creek/Morrison).
Toward the northeast and above the Morrison on the geologic column is the hogback of the Dakota sandstone [(f) on the drawing] that we see in so many of the areas covered by the Hayden Survey in Colorado. This is a hard, resistant sandstone that was originally beach sand deposits now well cemented by tough silica. One of the farthest northeast hogbacks in the scene is the Niobrara limestone formation [(g) on the drawing]. There are also other formations that occur here with less drama, such as the Carlile, the Pierre, the Fox Hills, and the Laramie. This entire sequence of sedimentary rocks with its alternating hogbacks and valleys reminds one of an echelon formation of military precision written on the land. As the scene trends more and more toward the northeast, the gravels that are the detritus from more recent erosion starts to cover the formations of the older rocks, at least until we hit the remnants of the next episode of mountain building.
At (i) on the drawing, we can see the dramatic impact of the Laramide orogeny in the horizontal strata of the conglomerate/sandstone of the Dawson formation that form Dawson Butte and many of the other buttes around the Larkspur area including Raspberry Butte, from which this drawing was made. But the Dawson is less resistant to erosion than its Fountain formation counterpart, so only those portions protected by erosion-resistant cap rock still stand high. Dawson Butte is capped by the Wall Mountain tuff—a welded tuff that flowed here from some 120 miles away as Wall Mountain in the Collegiate Range near Mt. Princeton had a cataclysmic eruption about 37 million years ago. That massive geologic event sent pyroclastic debris across large parts of central and eastern Colorado. It is this tuff that the section’s opening quote describes.
Two other resistant cap rock formations also top some of the buttes in the area around Larkspur. The older of the three cap rock formations is the Larkspur conglomerate, recognized by its stratigraphic position below the Wall Mountain tuff and by its pinkish clasts of Pikes Peak granite that are imbedded in its rock matrix. The youngest of these three cap rocks is the Castle Rock conglomerate, which lies just above the Wall Mountain in the geologic column. It gets its name from the rock atop the major butte that overlooks the town of Castle Rock that lies just beyond the right side of the scene.
Except for the more recent Dawson formation and the newer gravels, the sedimentary rocks in this landscape dip toward the northeast. Pleasant (Perry) Park is the far southwestern edge of a large structural depression called the Denver Basin. This huge landform underlies nearly all of the northeastern one-fifth of Colorado and contains some of the largest fossil fuel deposits in the state. By some estimates the Denver Basin has over 28,000 oil and gas wells and a mostly unmined collection of coal measures. “Coal measures” is the term the survey used for any sedimentary rock formation that contained any recognizable coal deposits, from lignite to anthracite. Geochemically, the type of hydrocarbon produced in geologic formations depends to a great extent upon the environment in which the original organic matter that eventually created the fossil fuel was produced. By and large, oil/petroleum develops in paleomarine environments where plankton, phytoplankton, and other marine organisms lived. Natural gas, however, develops mostly from terrestrial organic matter that comes from forests and forbs. Coal is also a mainly terrestrial organic matter by-product.
In the Pleasant Park landscape we literally can see the alternating terrestrial, littoral, and marine environments locked into the rock formations. It is no coincidence, then, that the Denver Basin is known for both oil and gas production. Coal is a lesser player here, but we can still see its impact from the drawing. The dotted line (h-h) on the depiction indicates a minor outcropping of coal strata that runs across nearly the entire scene. Most of these coal deposits are not well enough developed or large enough to be mined profitably, but coal is in evidence nonetheless.
There are few if any oil or gas wells in Perry Park to bring wealth to the area. The current wealth here comes from another source—commuting corporate executives, engineers, airline pilots, and real estate developers. This part of Douglas County has become a land of the upper-middle class, exurban, and horse ranchette. Most of the buildings in the photograph are part of what we could refer to as “equestrian properties.” These spreads of five to several hundred acres cover the scene with low-density development including white fences, riding trails, horse corrals, show arenas, or all of the above. For an idea of the socioeconomic status of the people who populate the scene today, you merely need to look at the most recent US census for the area; the median family income and the median housing price for the area are both nearly double that for the rest of the state. There are PGA-sanctioned golf courses just out of view with new country clubs springing up seemingly every year. The park is within easy commuting distance to both Denver and Colorado Springs, so the residents have access to the urban scene and live in an environment that is pastoral and exclusive. It would be reasonable, one would think, to regress back to the former name of the area: Pleasant Park. It is certainly a pleasant place to live while shunning the crowded cityscapes that provide economic benefits for the scattered residents who occupy the land.