INSIGHTS INTO ALPINE MORAINE DEVELOPMENT FROM COSMOGENIC CL-36 BUILDUP DATING

We have used cosmogenic Cl-36 surface exposure dating to determine app arent construction ages of late Pleistocene moraines in the Sierra Nev ada, the White Mountains, and the Wind River Range, all in the western United States. The inferred glacial chronologies from the various loc alities all exhibit certain characteristics: (1) Local records are fra gmentary and deposits of some glacial advances are always missing; no location has deposits of all glaciations and no glacial advance is rec orded at all locations. This is due either to unfavorable conditions f or glacier development at some times or to obliteration of earlier dep osits by later, more extensive glaciers. (2) Most moraines have young apparent exposure ages, with only a few older than the last glacial cy cle. This may be due to young true ages of these deposits, erosion of moraine surfaces, or obliterative overlap and covering of older deposi ts by younger ones. (3) Many deposits that were previously correlated (e.g., based on their stratigraphic positions) are not correlative; th ey may represent different stades and, sometimes, even different glaci ations. Similarly, some previously uncorrelated deposits have the same exposure ages and may be correlative. (4) Clusters of several distinc t moraines of nearly the same age are present at most locations. These clusters suggest that alpine moraines are formed during short deposit ion episodes that last between several hundred and several thousand ye ars.