About 50 million years ago, during the Eocene epoch, the young Rocky Mountains were almost finished growing, and the landscape of what is now parts of Colorado, Utah and Wyoming consisted of rugged mountains separated by broad intermountain basins. Rains falling on the slopes of these mountains ran off of the land and collected into streams that carried sand, silt, mud, and dissolved materials down into the lakes that occupied the intermountain basins. Over time, these sediments began filling the lakes, and many types of fossils were preserved within them.
Abundant plants and algae grew on the margins of these lakes, providing a perfect habitat and food source for Elimia Tenera, the freshwater snail. When the snails died, their shells sank to the bottom of the lake. The snails were so prolific that entire lenses of sediment were composed almost entirely of their shells.
After these layers were buried, groundwater moved through the sediments. Small amounts of microcrystalline silica that were dissolved in the groundwater began to precipitate, possibly in the form of a gel, within the cavities of the smail shells and the emply spaces between them. Over time, the entire mass of fossils were silicified, forming the brown fossiliferous agate (also known as chalcedony) that we know today as Turritella Agate.
Found in Utah, Wyoming and Colorado.