Clastic sedimentology is Dr. Triplehorn's major area of emphasis. His past research has focused on clay mineralogy, fluvial sediments, glauconite, diagenesis and shale petrology. In recent years his focus has been on volcanic ash partings in coals; these have opened a surprising variety of new avenues of research. Coal-bearing strata in Alaska, Washington, and throughout the Rocky Mountain states have been dated by K-Ar and fission-track methods. Criteria have been developed for recognition of volcanic ash partings; with these criteria their abundance has been found to be much greater than previously suspected. New occurrences of relatively rare aluminum phosphate minerals have also been found in many ash partings in Alaska as well as a few localities in Colorado and Kentucky. Ash partings can provide a stratigraphic framework that permits the interpretation of the development of coal-producing environments through time as well as detailed coal correlations and rates of sedimentation. Because both coal formation and volcanism were common and widespread here during the Mesozoic and Cenozoic, Alaska may be one of the best places in the world to pursue these studies. In the late 1980's, emphasis shifted from volcanic ash partings to the similar fall-out material from the end-of-Cretaceous meteor impact. This work has been with the U.S.G.S. personnel in Denver and has resulted in the discovery of shock-metamorphosed quartz and two new localities.