Geologic History of Sylvan Dale Ranch
Sylvan Dale Guest Ranch is a great place to visualize geology: Earth history and processes. (Courses from universities in Boulder, Greeley, and Fort Collins use the Ranch for field trips, under the auspices of the Heart J Center for Experiential Learning.)
The pattern of geology provides the foundation for everything else, from vegetation to human use. Useful references on local geology are Chronic and Chronic’s Prairie, Peak, and Plateau and Chronic and Williams’ Roadside Geology of Colorado.
The three general classes of rocks are seen at Sylvan Dale: sedimentary sandstones, shales, and conglomerates make up Red Ridge; igneous granite makes Green Ridge and parts of Alexander Mountain and “Inspiration Point” (a spur of Stone Mountain); and metamorphic gneisses and schists are exposed in the Big Thompson Canyon and Sulzer Gulch.
Several of the great ages of Earth are also exhibited. The Precambrian Era is represented by the ancient igneous and metamorphic rocks of Green Ridge and Alexander Mountain. The Paleozoic Era, the “Age of Fishes and Amphibians,” is represented by the oldest sedimentary rocks, the Fountain Formation. Most of our sedimentary rocks represent the Mesozoic Era, the “Age of Reptiles.” Cenozoic rocks—representing the so called “Age of Mammals,” from 65 to about 2 million years ago (mya)—are missing from the area, eroded to bits, hauled downstream, and gone to rest as sediments in the Gulf of Mexico. The last 2 million years, including the Pleistocene Ice Ages, are represented by unconsolidated sands, gravels, and soils.
From east to west (down-section, younger to older), the major rock formations exposed at Sylvan Dale are as follows:
The Dakota Group (pale tan, coarse sandstones) is often exposed in classic “hogback” ridges, typically supporting ponderosa pine (as on the east ridge of the Big Valley) are beach sands from the Cretaceous Period, 120 million years ago. The classic exposure of the Dakota in our area is the Devil’s Backbone, half-way between Sylvan Dale and Loveland. Part of the exposure is protected as a Natural Area by Larimer County Parks and Open Lands.
The Morrison Formation (grayish clay- and siltstones, exposed near the base of the east ridge of the Big Valley) is named for the town of Morrison, southwest of Denver. It dates from Jurassic Period, 170 mya, the interval of the age of dinosaurs when first birds evolved. The Morrison is the classic source of dinosaur fossils, and has produced them as nearby as Horsetooth Reservoir, west of Fort Collins. However, the Morrison exposed on the east side of Sylvan Dale’s Big Valley is a shallow marine sediment, yielding tiny fossil shells, but no dinosaurs.
The Lykins Formation (red shales and siltstones) is exposed on Red Ridge N of Big Thompson School. It represents the Triassic Period—marking “The Great Extinction,” the beginning of Age of Reptiles, 210 mya, the time at which first mammals evolved. Silty remains of a Sahara-like desert, the Lykins Formation makes weak redbeds that erode to produce longitudinal valleys west of ridges of the Dakota and Morrison formations and east of ridges of the Fountain and Lyons formations. Sylvan Dale’s “Big Valley” is an example, as is the eastern slope of Eagle Ridge (Waterdale Glade along County Road 29). These valleys often have deep soils and support irrigated haylands or pastures (or have been dammed to impound reservoirs, such as Carter Lake and Horsetooth Reservoir).
The Lyons Formation (sandstone, forming resistant cliffs, often sparsely clothed with ponderosa pine) is exposed atop Big T Red Ridge and on the Carter Valley Anticline (locally known as “Heritage Hill”) in the NW corner of the Big Valley. This formation dates to the Permian Period, 220 mya, during the Age of Amphibians. The Lyons represents thinly bedded dune sands, named from Lyons, Colorado, about 15 miles south of Sylvan Dale. Nearby Masonville and Arkins were established as quarry towns exploiting the flagstone of the Lyons Formation. There still are several active quarries near Sylvan Dale. The rock is frequently used to face buildings (see Sylvan Dale’s “Heritage”) and to pave patios, and once it was harvested in massive slabs to build sidewalks in towns along the Front Range.
The Satanka Formation (siltstone to fine-grained sandstone) comprises massive redbeds on Eagle Ridge and “Big T Red Ridge” also dates from the Permian Period, 270 mya.
The Ingleside Formation (calcareous red sandstone, fine-to medium-grained, well-sorted, cross-bedded) also dates back to the Permian. White streaks of calcium carbonate help to distinguish it from the overlying Satanka Formation.
The Fountain Formation (red conglomerates or coarse sandstone) is exposed at river level on the Main Ranch on the east side of the Big Thompson and also in “Puddingstone Park” in Sulzer Gulch, in contact with the granite of Alexander Mountain at the level of the Hansen Feeder Canal, and in “The Hideout,” S of US 34 on the way to Inspiration Point). The Fountain dates from the Pennsylvanian/Permian periods, 280 mya—the Age of Amphibians and the age of coal swamps. It was named from Fountain, Colorado, near Colorado Springs. This is the formation exposed as Boulder’s Flatirons, Morrison’s Red Rocks Park, and Garden of the Gods near Colorado Springs.
This rock is worth a close look. It is easy to see that it is made up of stream-worn pebbles and sand cemented together. These are eroded bits of an ancestral range of Rocky Mountains that stood 300 million years ago on roughly the alignment as today’s Rockies. Pick up a handful of sand and gravel from the bottom of the Big Thompson and you have a sample of modern material similar to that of the ancient streambed represented by the Fountain Formation.
By the way, the prominent chalky streaks in the Fountain are thought to represent root channels of ancient plants, tree-sized relatives of today’s horsetails (aka “scouring rush” or “joint grass”). In a wonderful connection with deep history, horsetails grow today in the coarse sandy soils developed from erosion of the Fountain Formation. Look for horsetails in Sulzer Gulch, Maitland Gulch (south of the Draft Horse Pen, across from the Ranch mailbox), and in The Hideout, south of US 34 on the way to Inspiration Point).
To review: Eagle Ridge and Red Ridge represent a major part of the Paleozoic Era and the east ridge of the Big Valley represents the Mesozoic, an interval of over 200 million years. This is the same sequence of rocks exposed in the famous I-70 road-cut at Morrison, west of Denver. The situation around Loveland is a little more complicated because of local faulting and folding. On the way to Loveland, we cross the sequence several times. The “Devil’s Backbone” is the easternmost ridge of Dakota Sandstone. The geologic map below is extracted from the USGS map of the Masonville Quadrangle.
Sylvan Dale is a wonderful place to imagine deep time. The oldest sedimentary unit in our area, the Fountain Formation, is 300 million years old. It lies atop Precambrian granite (the Boulder Creek granodiorite), which is 1.75 billion years old, some of the oldest rocks exposed in Colorado. Where you see this contact, imagine nearly 1.5 billion years of Earth history missing from the rock record. It’s like having a partial set of encyclopedias with volumes A, B, C and then Z.
From about 2 million to about 20,000 years ago, there were episodes of alpine glaciation in the Rockies, with glaciers flowing nearly to the elevation of Estes Park. Glacial intervals were wetter than the present and there was a lot of erosion. The Home Pastures below the Hansen Feeder Canal mostly are on gravels washed off Alexander Mountain at that time. West of the Office Parking Lot (and in the road-cut west of the Wagonwheel Barn) you can easily see the sorted terrace gravels, cobbles, and boulders of the Ice Age floodplain of the Big Thompson River.
The present Rocky Mountains were uplifted in two major episodes, the first about 65 mya, about the end of the Age of Reptiles, the second some 5 or 6 million years ago in the late Miocene and Pliocene. Looking south from Green Ridge, notice the angle of the sedimentary units. Extend that angle westward to imagine the original height of the Rocky Mountains before they were worn away by tens of millions of years of erosion.
Geologists use big numbers. It is not easy to deal with the concept of billion. A loose stack of used $1,000 bills an inch high would be about $100,000. $1 million in $1000 bills would make a stack 10 inches high. A billion is a thousand million. $1 billion in $1,000 bills would make a stack 10,000 inches high, which is about 833 feet, which is twice the depth of the Big Thompson Canyon below Inspiration Point! Conclusions? A billion years is a long time…and we can visualize it at Sylvan Dale…a scenic “textbook” of Earth history.