Most U.S. Geological Survey (USGS) work in the Sauk River quadrangle began in 1978 and has continued through 1991. It is part of a major USGS project to compile and map the Concrete 1 X 2 degree quadrangle at 1:100,000 scale. J.A. Vance mapped many areas throughout the quadrangle during the years 1951-1985. A.B. Ford and colleagues (see map credits) contributed mapping in much of the Glacier Peak Wilderness area during studies in 1980-1982 mandated by the Wilderness Act. D. B. Booth mapped during the years of 1978-1986, and R. W. Tabor mapped during the years of 1978-1991. R.A. Haugerud contributed mapping and considerable geologic insight. Responsibility for the bedrock mapping and compilation rests with Tabor, Booth, and Vance. Responsibility for mapping the unconsolidated deposits resides with Booth. Vance determined all new fission track ages (Table 3).

Our field work was greatly facilitated by the assistance of Kris Alvarez, Steven L. Garwin, Patrick Goldstrand, Kathleen Ort, Vicki Pease, Carolyn Ortenburger, Ron A. Sonnevil, Fred Zankowsky, and the late Carl Huie. We thank Michael Swanson, Fred Zankowsky, and especially Carolyn Ortenburger for office and laboratory services. R.W. Tabor prepared the digital version of this map with considerable help from Kris Alvarez, Kathleen Duggan, Tracy Felger, Eric Lehmer, Paddy McCarthy, Geoffry Phelps, Kea Umstadt, and Carl Wentworth. Paleontologists who helped immensely by identifying deformed, commonly recrystallized, and usually uninspiring fossils are Charles Blome, William Elder, Anita Harris, David L. Jones, Jack. W. Miller, and Kate Schindler. Chuck Blome has been particularly helpful (see Table 1 in the downloadable pamphlet text).

We thank John Whetten, Bob Zartman, and John Stacey and his staff for unpublished U- Pb isotopic analyses (cited in Table 2). Robert Fleck has generously shared unpublished Rb-Sr and K-Ar data with us. Dennis Sorg provided clean mineral separates for K-AR ages: Carolyn Ortenburger and Michael Ort extracted and analyzed most of the argon. P. Klock, S. Neil, T. Frigs, L. Espos, S. MacPherson, and M. Taylor analyzed the potassium (table 4). We have had fruitful discussions on the geology of the area and the North Cascades in general with Ned Brown, Bob Miller, the late Peter Misch, Greg Reller, Dave Silverberg, and John Whetten. Dave Harwood, Ralph Haugerud, and Ned Brown critically reviewed the manuscript and map. We are particularly indebted to Ralph for sharing his ideas on the origin of the onlap plutons of the granodioritic group.

Most of our helicopter support was in the capable hands of Anthony Reese. We also thank the late Jack Johnson for his flying skill. U. S. Forest Service personnel at Darrington and Verlot have been helpful in many ways; we especially thank Howard Barstow, Janet DeRoco, and Fred Schaub. Time scales used in this discussion and for the correlation chart on the geologic map are those of Berggren and others (1985) and Cowie and Bassett (1989). Igneous rock terminology is that of Streckeisen (1973, 1979).