USGS Visual Identifier
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Geologic Unit: Mesa Falls

Mesa Falls Tuff of Yellowstone Group (ID*,MT*)
Mesa Falls ash (informal) (CO*,ID*,NE*,WY*)
For informal usage, the rank or lithologic term should not be capitalized.


Geologic Province:
Snake River basin*
Green River basin*
Paradox basin*
Salina basin*
Montana folded belt province*
Yellowstone province*
Idaho Mountains province*
Areal Extent:

Type Locality:
Type section: roadcut on U.S. Highway 20 about 6.8 km north of Ashton, Fremont Co., ID. Named from Upper Mesa Falls, Henrys Fork of Snake River in the Island park area, Fremont Co., eastern ID (Christiansen and Blank, 1972).

Unit Name History:
Name proposed for the middle formation of Yellowstone Group; Isotopic dating; Principal reference (Christiansen and Blank, 1972).
Overview, downwind equivalent informally called Pearlette type S (Naeser and others, 1973; Love, Christiansen, 1980).
Areal limits, downwind equivalent (Pearlette type S) revised to informal Mesa Falls ash (Izett and Wilcox, 1982).

Note: If you need more information, please contact Nancy Stamm (
Asterisks (*) indicate usage by the U.S. Geological Survey. Other usages by state geological surveys.

"No current usage" implies that a name has been abandoned or that it has fallen into disuse. Former usage and, if known, replacement name given in parentheses ().

Slashes (/) indicate unit name usage violates the 1983 North American Stratigraphic Code. This violation may be explained within brackets ([ ]).

U.S. Department of the Interior, U.S. Geological Survey, Reston, VA, USA
Database project personnel
Last modified: 15-Jan-2013