Physiographic Divisions of Alaska By CLYDE WAHRHAFTIG GEOLOGICAL SURVEY PROFESSIONAL PAPER 482 A classzjfcation and brief descr$tion with a discwsion of high-latitude physiographic processes UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B . Nolan, Director Library of Congresa catalog-card No. GS 68-805 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS Page Pane Ab&ract------------------------------------------- 1 Description of the physiographic divisions-Continued Introduction------------------_-------------------- 2 Intermontane Plateaus-Continued Acknowledgments- - - - - - - - - - - - - - - - _ - - - - - - - - - - - - - - - - - 3 Western Alaska province-Continued The basis for the classification--- - - - -------- - - - -- -- -- - 3 Kuskokwim Mountains----- - ----_-_-- -- -- - Summary of the geologic history of Alaska ------------- Innoko Lowlands----------------------- Details of sculpture-physiographic evolution in the last Nushagak-Big River Hill8- - - - ----------- millionyears-----------..-------------------_----- Holitna Lowland........................ Glaciated highlands and mountains- - - - - -- - - - -_-- - Nushagak-Bristol Bay Lowland- - - - - -- - - - Glaciated lowlands---- - - - - - -- -- -- - -- -- -- - - -- - --- Seward Peninsula--------------------------- Unglaciated uplands- - - - - - - - - - - - - - - - - - - - - - - - - - - - BeringShelf----------------------------_-- Unglaciated lowlands----- - - - - - - - - - - - - - - - - - - - - - - - Yukon-Kuskokwim Coastal Lowland- - - - - - Description of the physiographic divisions- - - - - - - - -- - - - Bering Platform- -- -- -- ---- - --- ----- ---- Interior Plains---------------------------------- Ahklun Mountains--- -- ---- ----- --- -- -- - ---- Arctic Coastal Plain- - - .................... - Pacific Mountain Syatem- ....................... Rocky Mountain System- - - ---- -- - - - - -- -- - -- -- -- Arctic Foothills ---- -- -- - ---- - -- - ---- - -- -- -- - Alaska-Aleutian province- - - - - - - - - - - - - - - - - - - - Aleutian Islands--- -- ------ -- - Arctic Mountains province ---- - - - - -- -- - -- -- -- ---- -- -- -- De Long Mountains- - - - ---------------- Aleutian Range ............................ Noatak Lowlands--- --- -- - - -- ------ -- -- - Alaska Range (southern part) ----- ------- Baird Mountains- - ------------ --------- Alaska Range (central and eastern part),-- Central and eastern Brooks Range -------- Northern Foothills of the Alaska Range- - - Ambler-Chandalar Ridge and Lowland sec- Coastal Trough province- - - - - --------------- tion--------------------------------- Cook Inlet-Susitna Lowland- - - - ---- -- --- - - - - - Intermontane Plateaus- - -- - - - - - - - - - - - -- -- _ - - - -- - Broad Pass Depression- - - -- --- ----- Talkeetna Mountains- - -- - -- - -- -- ---- Northern Plateaus province- - _ - - - - -- - -_--- -- - - - - Porcupine Plateau- - _ - - - - - _ - - - - -- - - - - - -- Upper Matanuska Valley- --------------- Ogilvie Mountains- - - - - - - - - - - - - - - - - - - - - - Clearwater Mountains ---- - - - - - - - - - - - - - - - Tintina Valley ---------- ---------------- Gulkana Upland--- -- -- ---- ---- -- --- - -- - Yukon-Tanana Upland-- - -- -- -- - _ -- -- - - - Copper River Lowland ------------------ Northway-Tanacross Lowland--- - - -- -- - _ - Wrangell Mountains- - --- -- --- - -- -- -- -- - Yukon Flats section..................... Chatham Trough ....................... Rampart Trough- - - - - _ - - - - - - - - - - - - - - - - - Kupreanof Lowland---- - - - - - - - - - - - - - - - - - Kokrine-Hodzana Highlands- - - - - - -- - - - - - Pacific Border Ranges province ---- - -- --- -- -- - Western Alaska province- - - - --- --- - -_-- ----- Kodiak Mountains ...................... Kanuti Flats-- - - - - - - - - - - - - - - - - - - - - - - - - - Kenai-Chugach Mountains ---- --- -- - ----- Tozitna-Melozitna Lowland--- - - - - _ - - - - - - St. Elias Mwtains..................... Indian River Upland .................... Fairweather Range- - - - - - -- -- ------- Pah River section ....................... Gulf of Alaska coastal section- ----------- Koyukuk Flats......................... Chilkat-Baranof Mountains-- - -- -- ------- Kobuk-Selawik Lowland---- - - - - -- --- - - - - Prince of Wales Mountains- - - ----------- Selawik Hills-- -- - -- - --- - - --- -- - - -- - -- - - Coast Mountains- - -- - - - - - - - - - - - - - - - - - - - - - - - Buckland River Lowland- --------------- Boundary Ranges ---- -- - -- -- - -- -- - --- --- Nulato Hills- - - ........................ Coastal Foothills- - - - -- -- -- -- ---- -- -- -- - Tanana-Kuskokwim Lowland- - - - --- -- -- - References cited .................................... Nowitna Lowland------ - - -- -- -- - -- --- -- - CONTENTS ILLUSTRATIONS [Plates are in pocket] PLATE1. Maps of physiographic divisions of Alaska. 2. Photographs illustrating minor physiographic features and microrelief in Alaska. 3. Aerial photographs illustrating the physiography of the Rocky Mountain System and Intermontane Plateaus in Alaska. 4. Topographic maps and aerial photographs illustrating the physiography of the Interior Plains, Rocky Mountain system, and Intermontane Plateaus in Alaska. 5. Topographic maps illustrating the physiography of the Pacific Mountain System in Alaska. 6. Aerial photographs illustrating the physiography of the Pacific Mountain System in Alaska. FIGURES1-6. Map of Alaska showing- Page 1. Location of 1 :250,000 topographic maps- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .. - - - - - - - - - - - - - - - 4 2. Mean daily minimum temperatures in January ------ -- -- - -- - ---- -- - -- - -- -- - - -- -- -- -- -- - - -- -- - - - 9 3. Mean daily maximum temperatures in July ----- - -- - - --- -- -- - - -- -- - --- -- -- --- -- -- --- -- - -- -- 10 4. Average annual precipitation- - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - 11 5. Extent of existing glaciers, Pleistocene glaciers, and permafrost regions------ - -- -- - -- --- --- -- -- ---- 12 6. Drainage basins of major rivers .............................................................. 19 TABLE Pwe TABLE1. The geologic time scale................................... 6 PHYSIOGRAPHIC DIVISIONS OF ALASKA AHRHAFTIQ ABSTRACT glaciers, permafrost conditions, and geology of each section are Alaska occupies the great northwestern peninsula of North briefly described. America, which slopes and drains westward to the Bering and The Rocky Mountain System, Intermontane Plateaus, and Chukchi Seas. Most of the State is mountainous or hilly 81- Pacific Mountain System together constitute the North Ameri- though plains 20-100 miles wide abound. The central part, can Cordillera, one of the major physiographic features of the which slopes westward, consists of interspersed plains, plateaus, continent. Throughout most of its history the North American and rounded mountains, extending from beyond the Canadian Cordillera has been the site of geosynclinal sedimentation, border to the west coast; this part is bordered on the north dominantly miogeosynclinal with interbedded carbonate and and south by high rugged ranges which effectively cut off the well-sorted clastic rocks in the Rocky Mountain System and bulk of the peninsula from the Arctic and Pacific Oceans. The northern part of the Intermontane Plateaus and dominantly northern range is the Arctic Mountains province of the Rocky eugeosynclinal with interbedded volcanic and poorly sorted Mountain System, dominated by the Brooks Range whose sum- clastic rocks in the Pacific Mountain System and southern part mit altitudes are 6,000-8,000 feet. North of this province are of the Intermontane Plateaus. Orogenic activity, accompanied the Arctic Foothills, also part of the Rocky Mountain System, by the invasion of immense granitic batholiths in the Pacific and the Arctic Coastal Plain, the northwestern extension of Mountain System and Intermontane Plateaus, has affected the the Interior Plains. The southern mountain barrier, part of the North American Cordillera almost continuously since early Pacific Mountain System, is a pair of ridges separated by a Jurassic time. Near the end of Cretaceous time, orogenic ac- line of discontinuous depressions, the Coastal Trough province. tivity reached its climax; and most of Alaska was converted to The northern ridge of the pair is the Alaska-Aleutian province ; dry land, which has remained ever since. In Cenozoic time and the southern, the Pacific Border Ranges province. The Alaska has been subjected to faulting, warping, and local fold- highest peaks of North America, rising to an altitude of more ing. These processes formed highlands, whose erosion pro- than 20,000 feet, are here; and mountains 8,000-12,000 feet in duced large quantities of poorly consolidated sediments, and altitude are common. basins, in which these sediments were deposited and are now The part between the Arctic Mountains and the Paciflc preserved. Deformation continues and is particularly strong Mountain System is a disordered assemblage of flat plains and along the Pacific Coast. Active volcanoes are in the Aleutian rolling uplands, surmounted here and there by groups of low Islands, Alaska Peninsula, and Wrangell Mountains. mountains; the whole region declines in relief and altitude Alaska has bedrock structure