The collection of papers that follow continues the series of U.S. Geological Survey (USGS) investigative reports in Alaska under the broad umbrella of the geologic sciences. This series represents new and sometimes-preliminary findings that are of interest to Earth scientists in academia, government, and industry; to land and resource managers; and to the general public. The reports presented in Studies by the U.S. Geological Survey in Alaska cover a broad spectrum of topics from various parts of the State, serving to emphasize the diversity of USGS efforts to meet the Nation's needs for Earth-science information in Alaska. This professional paper is one of a series of 'online only' versions of Studies by the U.S. Geological Survey in Alaska, reflecting the current trend toward disseminating research results on the World Wide Web with rapid posting of completed reports.