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<br />38
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<br />C.F. Wa)'IJwma.r. R.D. JtJ,.r~lt I Geomorphology J I (1994) /'5'=40
<br />
<br />The T4 terrace and FB4 flood boulders represent the
<br />final phases of significant fluvial activity in Arthurs
<br />Rock Gulch. and provide evidence for limited channel
<br />incision and gravel deposition. FB4 flood boulders are
<br />evidence for a minor flood that occurred sometime after
<br />formation of the T4 terrace. The oldest trees growing
<br />on FB4 deposits range in age from 50 to 80 years old.
<br />Thus, the FB4 flood must be at least 50 years old but
<br />less than 297 years old.
<br />
<br />6.-conclusions
<br />
<br />We interpret the majority of coarse bouldery deposits
<br />found at elevations up to 10m above the modern chan-
<br />nel of Anhurs Rock Gulch as flood deposits. Other
<br />similar deposits are probably debris-flow deposits.
<br />Flash floods in Arthurs Rock Gulch capable of trans-
<br />porting large granodiorite boulders occurred at least
<br />five times during the late Quaternary. We have derived
<br />a preliminary flood chronology based on relative-age
<br />criteria, radiocarbon dating, tree-coring, and strati-
<br />graphic relations (Fig. 12).
<br />Relative-age estimates for FBw, FBO, and FB I
<br />deposits indicate that these deposits may be close in
<br />age provided the relative-age criteria is suitable for
<br />indicating differences in age among these deposits.
<br />However, the morphostratigraphic position of FB I
<br />deposits (Sites 3, 4, 5) indicates that they are younger
<br />than FBw and FBO. Age estimates for FBw, FBO, and
<br />FB I are based on tentative correlation with erratic boul-
<br />ders on Pinedale moraines. These correlations suggest
<br />that the FBw, FBO, and FB r flood boulders are aileast
<br />40.000 yr. old. Furthermore. FBO and FB I flood boul-
<br />ders (Sites 3, 4) overlie oxidized TO and TI terrace
<br />gravel and deeply-weathered colluvial deposits with
<br />mature soils that have thick Bt horizons (Table 3).
<br />This degree of soil development suggests that the col-
<br />luvium, and possibly the overlying TO and TI terrace
<br />deposits as well, may be at least 100,000 years old and
<br />possibly older (Shroba and Birkeland, 1983).
<br />FBw granodiorite boulders at Site 5 cannot be dif-
<br />ferentiated from FBO deposits on the basis of surface
<br />weathering features. It is not known if FBw and FBO
<br />deposits formed during the same or different floods.
<br />FB2 deposits at Site I are stratigraphically beneath
<br />alluvium dated to ca. 4500 yr BP. FB2 deposits.
<br />(excluding Site 1) show weathering characteristics that
<br />
<br />are similar to the weathering of surface erratics On
<br />early-middle Holocene moraines above treeline in the
<br />Colorado Front Range (Benediet. ]981; Dowdeswell.
<br />1982). The stratigraphic position and weathering of
<br />FB2 deposits indicates that they probably fOnned
<br />between 5000 and 10.000 yr BP. FB3 deposits are radi-
<br />ocarbon dated to about 300 yr. BP. FB4 deposits are at
<br />least 50 years old, but not older than about 300 yr BP.
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<br />Acknowledgements
<br />
<br />Funding for this study was provided by the U.S.
<br />Bureau of Reclamation and the U.S. Geological Sur-
<br />vey. We thank the staff of Lory State Park for permis-
<br />sion to conduct this study within the confines of the
<br />park. A. Duran assisted with preparation of the figures.
<br />We acknowledge the thoughtful comments and discus-
<br />sion provided by G.P. Williams. W.K Osterkamp, J.D.
<br />Smith, G. Leavesley. P.A. Carling. J.E. Costa, R.B.
<br />Webb. D.S. Kaufman, and two anonymous reviewers.
<br />
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<br />
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