When volcanic debris is ejected into the atmosphere it eventually settles back down to the surface as ash fall. Volcanic igneous rocks that result from this process are usually of uniform in grain size, which are tiny flakes of glass. Some ash ejected into the atmosphere may float for years before settling back down to the surface. Ash falls provide very good age datums that may be correlated for thousands of miles in some cases. A very important ash fall deposit in Colorado is the Lava Creek B ash that was created by Yellowstone Caldera around 640,000 years ago. This ash has been instrumental in providing age constraints on numerous formations around Colorado, such as the Brown's Park Formation.
Below: An example of ash fall deposits in Huerfano County.
Colorado Geological Survey