On the premise that sequential changes in varve thickness offer a proxy for climatic variations, we investigated varve thickness in three core segments from the distal lacustrine oil shales (Tipton and Laney members) of the Green River Formation, by means of an image analysis program. Of two strong bimodal periodicities one, at 4.8-5.6 years, is interpreted as an El Nino type (ENSO) phenomenon of atmospheric dynamics, while the other, at 10.4-14.7 years, is interpreted as the sunspot cycle, originally recognized in this formation by Bradley (1929, 1931). Weaker periodicities may exist at ca. 8 and 33 years--the latter also recognized by Bradley. Taken in conjunction with the work of Bradley (1929, 1931) and of Crowley et al. (1986), this suggests that some but not all of the oil shale of the Green River Formation is truly varved and can be used to infer climatic time-series. Automated instrumental scans have greatly extended the possibilities of varve studies as approaches to climatic variations in time, but careful selection of facies in their geological framework is necessary.