Date of this Version A bed of vitric ash occurs 2 m above the
primary fossiliferous unit at Lonergan Creek Locality in the Ash
Hollow Formation on the north shore of Lake McConaughy, western
Nebraska. Zircon crystals recovered from the ash were dated
using the fission-track method. The grouping of youngest-age
euhedral grains among the largely detrital suite is interpreted
as the airfall population, with a fission- track age of 10.42 ±
2.5 Ma. Owing to uncertainties about the origin of this youngest
group, the age should be interpreted as a "detrital" date and
considered a maximum age for the ash. The Lonergan Creek Ash
cannot be correlated with certainty to any other dated Great
Plains ash. However, it is of similar age to an ash 37 m above
the base of the type section of the Ash Hollow formation and
also to the Davis Ash, lying above the Minnechaduza Fauna in
Cherry County, Nebraska.