An easy 2-mile loop trail located at JeffCo South Valley Open Space Park. Hike along the Coyote Song, Prairie Falcon, and Swallow trails to observe the unique geological formations of the area.
(Photo by John Meyer)
The Tilting Layers of South Valley Park
The famous flatirons of Colorado can be discovered at numerous iconic locations along the Rocky Mountain Front. While the most celebrated examples include the Flatirons of Boulder, Red Rocks Amphitheater near Morrison, Roxborough State Park west of Castle Rock, and the Garden of the Gods near Manitou Springs, South Valley Open Space Park is an incredible hidden gem. As one of the lesser-known locations where these dramatic flatirons can be found, it provides an excellent hiking and learning opportunity very close to Southwest Denver.
If you look around this beautiful valley, you are standing on a spectacular geological frontier. The massive rock slabs shooting out of the ground here exist because of a powerful combination of ancient environments, tectonic forces, and millions of years of relentless weathering. The story of this landscape is written in four distinct chapters, each represented by a unique layer of rock beneath your feet.
The Fountain Formation (300 Million Years Ago)
- The Rock Unit: Distinctive red sandstone and conglomerate (rock containing mixed pebbles and gravel).
- The Environment: Long before the modern Rocky Mountains existed, an entirely different mountain range called the Ancestral Rockies stood just to the west. At this time, the land was high, dry, and home to prehistoric giants, including millipedes the size of snowboards and 30-inch-long dragonflies.
- How it Formed: Powerful, gravelly streams flowed out of these ancestral mountains. As the ancient peaks eroded, torrents of water carried loose sand and large granite and quartz cobbles down into wide riverbeds. Over millions of years, these heavy layers of debris compressed into stone. The signature deep-red color comes from a high concentration of iron rusting within the rock. This tough, stubborn formation is what creates the iconic, erosion-resistant "flatiron" slabs of the area.
The Lyons Formation (280 Million Years Ago)
- The Rock Unit: Flat, cleanly sheeted, salmon-colored sandstone.
- The Environment: As the Ancestral Rockies continued to wear down, the environment shifted dramatically. During the era of the supercontinent Pangea, North America became exceptionally arid. Vast, dry seas of windblown sand—strikingly similar to the modern Sahara Desert—blanketed much of what is now the American West.
- How it Formed: Relentless winds constantly built, shifted, and wore away massive sand dunes. Over time, these thick layers of desert sand were deeply buried, compressed, and tightly cemented together by quartz. This created the remarkably uniform, flat sheets of stone we see today, which are highly visible along the valley trails.
The Lykins Formation (260 to 250 Million Years Ago)
- The Rock Unit: Thinly bedded, brick-red siltstones and mudstones.
- The Environment: By the late Permian and early Triassic periods, erosion had reduced the old ancestral mountains to lowlands. The region transformed into a vast coastal plain and a shallow, highly salty (hypersaline) inland sea along the edge of Pangea.
- How it Formed: Fine, windblown silt and atmospheric dust settled into the calm, shallow waters, trapping layers of mud. Because the Lykins Formation is composed of fine muds and silts rather than hard sandstones, it is incredibly soft. It sits squarely between the tough, weather-resistant Lyons Sandstone and the Dakota Sandstone. Because it erodes so easily, it has completely washed away in places, forming the lush, green, and forested strike-valleys that sit quietly right behind the prominent ridges.
The Dakota Formation (100 Million Years Ago)
- The Rock Unit: Hard, light-colored quartz sandstone and shale.
- The Environment: In the Cretaceous period, and South Valley was completely transformed once again. A wide, warm, salty inland sea (the Western Interior Seaway) spread across the middle of North America. Dinosaurs actively roamed the muddy coastlines, backed by a dense shoreline forest of ferns and primitive trees.
- How it Formed: Rivers flowing from new western highlands brought massive amounts of sand and gravel to the coast, creating wide marine beaches. As the shoreline shifted, these beach sands hardened into the highly durable Dakota Sandstone. The muddy, marshy lagoons along this ancient coast provided the absolute perfect conditions for preserving the fossilized tracks of dinosaurs as they walked along the water's edge.
The Golden Fault & The "Drawbridge" Effect
Originally, all four of these formations were deposited sequentially as flat, horizontal layers on top of one another. They would still be buried deep underground today if it weren't for an immense tectonic event.
Between 68 and 40 million years ago, forces deep within the Earth violently pushed the modern Rocky Mountains upward. This intense pressure caused a massive break in the Earth's crust known as the Golden Fault.
As the land west of the fault line thrust skyward, it caught the edges of these flat sedimentary layers and forced them sharply upward. Much like a giant drawbridge tilting open to let a ship pass, the horizontal layers of the Fountain, Lyons, Lykins, and Dakota formations were forced into steep, dramatic angles pointing toward the sky.
Over the last several million years, wind, water, and ice scoured the valley. The elements easily washed away the soft muds of the Lykins Formation, leaving the incredibly tough sandstones of the Fountain and Dakota formations standing proud as the towering flatirons and steep "hogback" ridges that define South Valley Park today.
Logging Requirements
To log a find for this EarthCache, you must physically visit the coordinates, observe the local geology, and send your answers to the two field-based questions below to the Cache Owner via message or email. Please do not post your answers in your public online log. Also, feel free to post photos of yourself, your group, or a favorite location with your online post (not a requirement), but please ensure your photo does not inadvertently spoil the answers to the Questions.
Question 1: Textural Evidence of Ancient Rivers Approach one of the exposed red sandstone walls of the Fountain Formation near one of the Obsertation Points. Rub your hand safely along a clear section of the rock face and look closely at its composition.
- (A) Describe the texture of the stone—is it uniform and smooth like fine beach sand, or is it rough, gritty, and filled with mixed sizes of debris?
- (B) Find an embedded "cobble" or pebble trapped within the rock matrix. Estimate its size (in inches or centimeters) and describe its color and shape.
- (C) Based on your observations, how does this rough mixture of debris support the claim that these rocks were formed in high-energy, ancient riverbeds rather than a calm, quiet sea?
Question 2: Deciphering the Tilt & Erosion Stand back and look up at the massive flatiron structures towering above the trail.
- (A) Which cardinal direction (North, South, East, or West) is the rock face sloping down toward? Are they all sloping the same direction? What does this indicate in regard to the various layers?
- (B) Turn your gaze sideways to look at the sharp profile edge of the formation. Estimate the angle of the tilt relative to the flat ground (e.g., is it closer to a gentle 15°, a steep 45°, or a near-vertical 80°)?
