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Paleoclimatic Impications of the Alluvial Stratigraphy The alluvial stratigraphy in the main trunk drainages of the Piceance Basin have a similar sequence which includes three distinct periods of fill and one cut terrace (Figure 3). The larger first order tributaries of these streams more often contain two fill sequences correlatable to the first and last periods of deposition in the trunk streams. It is noted that the alluvial sequences of the lesser tributaries of the trunk drainages are fundamentally different for ephemeral tributaries draining the north flank highlands as opposed to the south flank highlands feeding the trunk streams. Those on the north flanks have proportionately more sediment to contribute to the major trunks because of the factors of insolation and slope stability mentioned above. The alluvial depositional sequence in the gulches in the vicinity of the study area is contained inside the Late Pleistocene dissection. (Dissection, as used here, defines excavation of bedrock during glacial or associated pluvial conditions while incision defines large-scale re-excavation and reworking of sediment deposited since the end of the glacial or pluvial period.) Dissection and incision are both considered to be the result of cooler, moister climates; avulsion is the term used to describe channel rework and rill formation in unconsolidated alluvium, which form as a consequence of one or a series of closely spaced heavy precipitation events. The first fill sequence is equivalent to Leopold and Miller’s (1954) Kaycee alluvium and consists of braided stream deposits; more recent work (e.g., Miller 1992, in prep.) provides ages of this deposit from over 13,000 uncalibrated radiocarbon years before present (rcybp) to about 7500 rcybp. The second fill sequence is thought to be equivalent to Scott’s Figure 3. Diagrammatic cross-section of the alluvial stratigraphy in Ryan Gulch. 9