Geologic map of the Duncan Peak and southern part of the Cisco Grove 7 1/2' quadrangles, Placer and Nevada Counties, California

This map covers an area of 123 km² on the west
slope of the Sierra Nevada, an uplifted and west-tilted
range in eastern California (fig. 1). The area is located
20 km west of Donner Pass, which lies on the east escarpment of the range, and about 80 km east of the Great
Valley Province. Interstate Highway 80 is the major route
over the range at this latitude and secondary roads, which
spur off from this highway, provide access to the northern
part of the area. None of the secondary roads crosses
the deep canyon cut by the North Fork of the American
River, however, and access to the southern part of
the area is provided by logging roads that spur off from
the Foresthill Divide Road that extends east from Auburn to the Donner Pass area (fig. 1).