Ž . Geomorphology 36 2000 19–45 www.elsevier.nlrlocatergeomorph Evolution of desert colluvial boulder slopes Donald A. Friend a, ) , Fred M. Phillips b , Sean W. Campbell c,1 , Tanzhuo Liu d , Pankaj Sharma e a Department of Geography, Minnesota State UniÕersity, Mankato, MN 56001, USA b Department of Earth and EnÕironmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801,USA c Department of Geography, Arizona State UniÕersity, Tempe, AZ 85287,USA d Lamont-Doherty Earth ObserÕatory, Columbia UniÕersity, Palisades, NY 10964,USA e Purdue Rare Isotope Measurement Laboratory, Department of Physics, Purdue UniÕersity, West Lafayette, IN 47907,USA Received 25 February 1999; received in revised form 1 February 2000; accepted 27 April 2000 Abstract We employed rhyolite domes formed during the past million years in eastern California to study the evolution of Ž . boulder-dominated slopes. The slopes studied are steep from ;258 to ;358 and are made of coarse, boulder-sized blocks. These slopes include well-varnished vertically-oriented colluvial deposits that appear similar to relict periglacial stone stripes. In this case, they are the result of ongoing desert slope processes. Such deposits are common throughout the arid southwestern US, but their morphometric character, fabric and rates of formation have not been assessed systematically. Basic field and isotopic dating methods are combined with two relatively new surface dating techniques, cosmogenic chlorine-36 and rock varnish microlaminae, to produce a detailed million-year-long slope development history with broad implications for geomorphic theory. Grain size, grain shape, grain angularity, and slope deposit area do not change significantly from genesis to ;0.5 Ma; trends in the data change markedly after that time. A geomorphic threshold occurs between ;0.5 and ;1 Ma. as indicated by significant changes in all parameters but grain orientation data. On-going erosion over the million-year continuum is evidenced by active debris flow features, by terminal Pleistocene ages of microlaminae on all deposits, and by 36 Cl ages that are progressively younger than potassium–argon ages for slope genesis. q 2000 Elsevier Science B.V. All rights reserved. Keywords: boulder fields; desert slopes; surface exposure dating; chlorine-36; varnish microlaminae; colluvial boulder deposits; slope processes ) Corresponding author. Tel.: q 1-507-389-2618; fax: q 1-507- 389-2980. Ž . E-mail address: friend@mnsu.edu D.A. Friend . 1 Present address: Department of Geography, University of Arkansas, Fayetteville, AR 72701, USA. 1. Introduction An important goal of geomorphology is under- standing the dynamics of landform evolution, and within geomorphology, the development of slopes has long been of great interest to modern and classic 0169-555Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-555X 00 00045-3