Molas Formation
The blood-red fossil soil at the base of the Pennsylvanian — a terra rossa and paleokarst fill marking tens of millions of years of missing time on the dissolved Leadville Limestone.
Also known as: Molas
Pennsylvanian — the great coal swamps
The world back then
Early Pennsylvanian, roughly 315 million years ago — but the real story here is the tens of millions of years of nothing that came just before. The old Mississippian sea floor sat bare and tropical, dissolving in the rain into a maze of caves, sinkholes, and blood-red soil, while dust blew in on the wind and filled the holes. No sea, no forest, no shade — just red dirt, bare limestone towers, and a hard equatorial sun over a slowly rotting landscape.
Now it's a thin red seam near the bottom of the canyon, the kind of thing you'd step over without a thought. But that seam is a paused clock — a stretch of deep time when this place simply stopped keeping records and turned to rust. Getting to stand on tens of millions of missing years, well-fed and idly curious, is a strange and enormous piece of luck.
Recognize it in the field
- Typical color
- blood-red to maroon, with gray-green mottling
- Weathers as
- A thin, recessive red slope or seam near the base of the canyon — soft, iron-stained soil and rubble that weathers back beneath the resistant limestones above and below.
- Grain
- clay and silt, with chert rubble
Look for
- blood-red to maroon iron-oxide-rich siltstone and mudstone (a fossil soil, or terra rossa)
- chert rubble and cave-fill inherited from the dissolved Leadville Limestone below
- wind-blown silt (loess) — the oldest known Paleozoic loessite in North America
- a thin unit marking a major unconformity — tens of millions of years of missing time
How this rock was made
A long-exposed tropical karst plain on the Leadville Limestone, weathering under a seasonal wet-dry climate near the equator as the Ancestral Rocky Mountains began to rise.
The setting: a residual soil (terra rossa) and cave-fill on a weathered limestone landscape, buried by wind-blown silt and later stream deposits (paleosol / paleokarst / loessite / fluvial)
Thin and variable; the basal Coalbank Hill Member is the terra-rossa paleosol, thickening into pockets and caves dissolved in the Leadville below.
In the rock record
↑ Younger — rests above
Where to see it
Regions: Paradox Basin / San Juan region (Four Corners)
- Molas Lake / Molas Pass (US-550, San Juan Mountains, Colorado)
- the Needle Mountains, Colorado
- Glenwood Springs, Colorado