Browns Park Formation
The loosely cemented, tuff-rich Miocene fill that once buried the worn-down Uintas — the open country of Browns Park, resting on the basement across an angular unconformity.
Also known as: Browns Park, Brown's Park Formation
Neogene — the modern world taking shape
The world back then
Miocene, maybe 15 million years ago. The old Uinta mountains have worn down to stumps, and loose sand and volcanic ash are burying what's left — a whole range slowly vanishing under its own gravel in a dry, open basin. It's a landscape being quietly erased, going flat. Later the rivers will change their minds and dig it all back out.
That's roughly what the river is doing beneath you right now — re-excavating a buried world while you ride through the hole it's cutting, doing nothing. It took tens of millions of years of burying and un-burying to reopen this country; you get it for the price of a shuttle and an afternoon. The rock does the digging; you do the looking.
Recognize it in the field
- Typical color
- pale tan to buff and white
- Weathers as
- Slope-forming, rounded soft badlands — the open, grassy country of Browns Park.
- Grain
- fine to coarse; loosely cemented
Look for
- loosely cemented, tuff-rich sandstone and conglomerate
- laps high onto the worn-down range
- rests on the basement across a sharp angular unconformity
- the youngest formation in the Dinosaur area
How this rock was made
A Miocene aggradational fill that buried the eroded Uinta uplift before renewed incision re-excavated the canyons.
The setting: river (fluvial), wind-blown (eolian), and lake-bottom (lacustrine) basin fill (tuffaceous)
Up to ~1,500 ft where preserved.
In the rock record
↓ Older — sits beneath
Where to see it
Regions: Browns Park / eastern Uinta flank (Middle Rocky Mountains province margin)
- the open country of Browns Park, above the Gates of Lodore (Green River)
Float past it: Browns Park · Lodore Canyon