Earth History at Sylvan Dale Guest Ranch

Sylvan Dale Guest Ranch is a great place to visualize patterns and processes of Earth’s history. Earth science students from local middle and high schools as well as the University of Colorado, the University of Northern Colorado, and Colorado State University often use the Ranch for field trips. Patterns of geology are the foundation for everything else, from vegetation to human land use to the awe-inspiring scenery.

Several of the great ages of Earth are on display at Sylvan Dale.

The granite and schist of Green Ridge, Inspiration Point, and Alexander Mountain represent the Precambrian Era.

A view across 1.8 billion years:

A view across 1.8 billion years: from Ice Age Little Canyon of the Big Thompson
to Pre-Cambrian rocks of Alexander Mountain. ⇱

The Paleozoic Era, the “Age of Fishes and Amphibians,” is represented by the Fountain Formation, the oldest of local sedimentary rocks. Most of our sedimentary rocks are from the Mesozoic Era, the “Age of Reptiles.” Cenozoic rocks, from the “Age of Mammals,” are missing from our immediate area, eroded to sandy bits, hauled downstream, gone to rest in the Gulf of Mexico. The last 2 million years, including the Pleistocene “Ice Ages,” are represented by sands, gravels, and soils.

eagle-ridge-alexander-green-ridge

View across the Home Pastures from Mesozoic rocks of Eagle Ridge
to Pre-Cambrian Rocks of Alexander Mountain and Green Ridge (right). ⇱

From east to west “(down-section,” younger to older), the major rock formations exposed at Sylvan Dale are as follows:

The Cretaceous Dakota sandstone often stands as pale tan “hogback” ridges, typically clothed with ponderosa pine. The well-known “Devil’s Backbone” between Loveland and Sylvan Dale is a classic exposure of the Dakota. The east ridge of Sylvan Dale’s Big Valley is another example. The Dakota represents beach sands from the sea that covered the Great Plains some 120 million years ago (mya).

The Morrison Formation (grayish clay- and siltstones, exposed near the base of the east ridge of the Big Valley) was named for the town of Morrison, west of Denver. Dating to the Jurassic Period, 170 mya, the Morrison is the classic source of dinosaur fossils. Dinosaurs have been collected as near to Sylvan Dale as Masonville. However, the Morrison on the east side of “the Big Valley” is a shallow marine sediment, yielding tiny fossil shells, but no dinosaurs!

The Lykins Formation consists of red shales and siltstones, exposed on Sylvan Dale’s “Big T Red Ridge,” north of Big Thompson Elementary School. It represents the Triassic Period—marking “The Great Extinction,” the beginning of the Age of Reptiles, 210 mya. These rocks are the silty remains of a Sahara-like desert. The Lykins is weak and erodes 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, along County Road 29. These valleys often have deep soils and support irrigated hayfields or pastures (or have been dammed to impound reservoirs, such as Carter Lake, Horsetooth Reservoir, and Green Ridge Glade Reservoir).

The Lyons sandstone forms resistant cliffs, often sparsely covered with ponderosa pine. The Lyons caps Sylvan Dale’s Eagle Ridge, Big T Red Ridge, and Heritage Hill (which overlooks the trout ponds and hayfields of the Big Valley). It represents the Permian Period, 220 mya, the Age of Amphibians. These are thinly bedded dune sands. The formation is named for Lyons, Colorado, about 15 miles S of Sylvan Dale. North of Sylvan Dale, Masonville and Arkins were quarry towns exploiting this flagstone, frequently used to face buildings and pave patios. Once it was once used for sidewalks in towns along the Front Range. Railroad spurs linked the quarries to urban markets.

The Satanka Formation (siltstone to fine-grained sandstone) forms the massive redbeds of Eagle Ridge”. Dating from the Permian Period, 270 mya, this rock provides inspiring scenery for painters and photographers!

The Ingleside Formation (calcareous red sandstone, fine-to medium-grained, well-sorted, cross-bedded) also dates from the Permian; white calcium carbonate streaks help to distinguish it from the overlying Satanka Formation.

The bottom of the sedimentary sequence at Sylvan Dale is the Fountain Formation. This coarse red sandstone (arkose) is exposed at river level just east of Jessup Lodge and in “Puddingstone Park” in Sulzer Gulch. It also is in contact with the granite of Alexander Mountain along the Hansen Feeder Canal, and in “The Hideout,” S of US 34 on the way to Inspiration Point). The Fountain represents Pennsylvanian to Permian periods, 280 mya—the Age of Amphibians (and the age of coal swamps). The formation is named from Fountain, Colorado, near Colorado Springs, this is the rock of Boulder’s iconic Flatirons and Morrison’s Red Rocks Amphitheater.

This Fountain is worth a careful look. It is easy to discern that it is made up of stream-worn pebbles and sand cemented together. These are the bits and pieces of the Ancestral Rocky Mountains that stood 300 million years ago on roughly the same alignment as today’s Rockies.

The Fountain Formation 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—as you and your horse cross the Canal and head up Sulzer Gulch, for example—imagine nearly 1.5 billion years of Earth history missing from the rock record. It’s like having a set of encyclopedias with A, B, C and then Z. You can step over that “abyss” on a hike to Eagle Ridge, or Echo Rock, or Inspiration Point.

From about 2 million to about 20,000 years ago, there were episodes of alpine glaciation in the Rockies, with glaciers flowing eastward almost to the elevation of Estes Park. Glacial intervals were wetter times than the present and there was a lot of erosion. The Home Pastures mostly are on sands and gravels carried down from Alexander Mountain during that time. Just above Sylvan Dale’s Office Parking Lot (and in the road-cut west of the Wagonwheel Barn) you can see the sorted terrace gravels, cobbles, and boulders of the Ice Age Big Thompson River.