The morphology of altiplanation in interior Alaska

ABSTRACT Altiplanation (cryoplanation) terraces are conspicuous and widespread elements of upland landscapes in Alaska’s interior. They occur in staircase-like series, primarily along divides in ridge-crest positions, and as summit platforms. Flanks (side slopes) consist of a steep upper section composed of coarse clastic debris and amorphous solifluction forms, grading through a break in slope (the ‘solischrund’) to a gentler lower section (cryopediment) dominated by well-defined solifluction lobes and low-order hillslope hydrological networks. An individual terrace unit consists of a steep scarp (riser) and a subjacent gently inclined tread (platform). A sequential group or series of adjacent terrace units, descending from a summit platform along ridge crests, are referred to collectively as an altiplanorium. Altiplanation terraces are a prominent feature of the altitudinal zonal arrangement of landscape phenomena in interior Alaska, and are confined to the upland periglacial zone between 915 and 1675 m.a.s.l. They are positioned below the contemporary snowline. Three study areas are given detailed verbal, cartographic, and photographic description. These areas (Mount Fairplay, High Valley/Denali Mountain, and Goodpaster Uplands) could serve as type localities for altiplanation landform assemblages. An appendix identifies the locations of many other areas in the Alaskan interior containing well-developed altiplanation landforms, and demonstrates the widespread geographic distribution of these landforms.