Cryoplanation Terraces (CTs) are erosional landforms reminiscent of giant staircases, with alternating shallow sloping treads and steep scarps leading to extensive flat summits.
Author: Kelsey Elizabeth Nyland
Category: Electronic dissertations
Cryoplanation Terraces (CTs) are erosional landforms reminiscent of giant staircases, with alternating shallow sloping treads and steep scarps leading to extensive flat summits. CTs are associated with periglacial (cold and unglaciated) environments and are typically found in elevated positions on ridges and hillslopes. Despite identification and discussion in geomorphic literature as early as the 1890s in the Urals, and the early 1900s in Alaska and the Yukon Territory, a consensus still has not been reached on the processes involved in CT formation. Two hypotheses continue to receive support from different factions of periglacial geomorphologists: (1) CTs are controlled primarily by geologic structure; and (2) CTs are dominantly controlled by climate through nivation -- the erosion process suite associated with late-lying snowbanks. This dissertation addresses some of the long-standing questions surrounding CT formation related to the time-transgressive nature of terrace treads, CT exposure ages, and long-term erosion rates associated with nivation processes. Explicit field testing was conducted on CTs throughout eastern Beringia, including the Seward Peninsula and the Yukon-Tanana Upland. Many prominent researchers have noted the proliferation of well-developed, relict CTs in unglaciated Beringia, which has experienced periglacial conditions throughout most of the Quaternary Period. Research questions are addressed in four interrelated, but self-contained chapters. Chapter 2 is a detailed review of CT terminology, formation hypotheses, and global distribution. A bibliometric analysis shows that no particular papers or authors have played "gatekeeping" functions that could explain the lack of explicit field testing over the last 50 years. Chapter 3 uses spatial statistics to examine differences in relative weathering indices and finds that treads at Skookum Pass, Eagle Summit, and Mt. Fairplay likely formed through scarp retreat. Chapter 4 is a geochronology study using 10Be and 36Cl terrestrial cosmogenic nuclides to determine exposure ages across treads near Eagle Summit and on Mt. Fairplay and to estimate erosion rates. Boulder exposure ages across these surfaces are synchronous with cold-climate intervals, suggesting climatic influence. Chapter 5 describes an unmanned aerial vehicle (UAV) survey conducted in an active nivation environment of northwestern British Columbia. In this "naturally controlled field experiment," long-term denudation rates for nivation since the Last Glacial Maximum on Frost Ridge were calculated from incipient terraces nearing the size and morphology of CTs in unglaciated Beringia. Chapter 2 effectively summarizes the history and current state of research on CTs, including an inventory of existing evidence for strong climatic influences on CT formation. Previous research has documented CT treads cutting across geologic structures, statistically preferred poleward orientations of scarps, and subcontinental-scale elevation trends closely match the position of paleosnowlines. Results from field investigations on relict CTs and in an active nivation environment, conducted as part of this dissertation, lend additional support to the proposition that climate controls CT formation through the mass balance of late-lying snowbanks. New data presented here, utilizing contemporary technologies, indicate that (1) CT treads are likely time-transgressive features, forming under scarp retreat; (2) boulder exposure is synchronous with local glaciations; and (3) long-term operation of nivation processes can produce landforms approaching the typical size of CTs.