Sixty-two samples from well-established comagmatic granitoid sequences and certain unassigned formations and plutons of the central part of the Sierra Nevada batholith between latitudes 37° and 38° N. have been dated by the isotopic U-Pb method on zircon. The U-Pb ages indicate the following age distribution of the granitoids: (1) The axial part of the batholith is occupied by Cretaceous granitoid sequences that are progressively younger eastward over a 37-m.y. interval extending from about 125 m.y. to about 88 m.y. ago. (2) A single, but extensive, Triassic sequence with an optimum average age of about 210 m.y. is present in the east side of the batholith. (3) Plutons and granitoid sequences of Jurassic age, most of them with U-Pb ages between 186 and 155 m.y., occur in both margins and locally in the interior of the batholith. The distribution of Jurassic ages suggests that prior to the emplacement of the Cretaceous granitoids, Jurassic granitoids were widely distributed across the central Sierra Nevada but were not emplaced in a west-to-east succession as were the Cretaceous granitoids. Few of our ages fall between 155 and 125 m.y. However, a U-Pb age of 144 m.y. has been reported on the Sage Hen Flat pluton in the White Mountains, and U-Pb ages between 134 and 128 m.y. have been reported on remnants of older granitoids farther south in the Sierra Nevada, which are associated with roof pendants and septa. Also, numerous K-Ar ages on hornblende in the range of 152 to 131 m.y. have been reported on samples collected farther north along the west side of the batholith. The distribution of U-Pb ages is consistent with the interpretation that in the central Sierra Nevada, a belt of Cretaceous granitoids trending about N. 20° W. crosses a belt of Jurassic granitoids trending about N. 40° W. However, the U-Pb ages provide little support for the existence of five cyclic intrusive epochs for California and western Nevada. Comparison of the U-Pb ages on zircon with the K-Ar ages on biotite and hornblende shows generally good agreement for the younger granitoids but decreasing agreement for increasingly older granitoids. Most of the K-Ar ages on biotite and many on hornblende from older granitoids appear to have been reduced as a result of reheating by younger plutons. The dispersion of K-Ar ages reflects the complex structural and thermal history of the batholith.

The June 27, 1995, storm in Madison County, Virginia produced debris flows and floods that devastated a small (130 km 2 ) area of the Blue Ridge in the eastern United States. Although similar debris-flow inducing storm events may return only approximately once every two thousand years to the same given locale, these events affecting a similar small-sized area occur about every three years somewhere in the central and southern Appalachian Mountains. From physical examinations and mapping of debris-flow sources, paths, and deposits in Madison County, we develop methods for identifying areas subject to debris flows using Geographic Information Systems (GIS) technology. We examined the rainfall intensity and duration characteristics of the June 27, 1995, and other storms, in the Blue Ridge of central Virginia, and have defined a minimum threshold necessary to trigger debris flows in granitic rocks. In comparison with thresholds elsewhere, longer and more intense rainfall is necessary to trigger debris flows in the Blue Ridge.

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Four major storms that triggered debris flows in the Virginia-West Virginia Appalachians provide new insights into the role of high-magnitude, low-frequency floods in longterm denudation and landscape evolution in mountainous terrain. Storm denudation in the Blue Ridge Mountain drainage basins is approximately an order of magnitude greater compared to basins located in the mountains of the Valley and Ridge province. This difference is probably the result of higher storm rainfall from the Blue Ridge storms. Radiocarbon dating of debris-flow deposits in the Blue Ridge indicates a debris-flow return interval of not more than 2-4 k.y. in mountainous river basins. This finding, combined with measurements of basin denudation, suggests that approximately half of the long-term denudation from mechanical load occurs episodically by debris-flow processes. Although floods of moderate magnitude are largely responsible for mobilizing sediment in lowgradient streams, our data suggest that high-magnitude, low-frequency events are the most significant component in delivering coarse-grained regolith from mountainous hollows and channels to the lowland floodplains.

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