Cryoplanation Terraces: Indicators of a Permafrost Environment

Abstract

Cryoplanation terraces are bedrock steps or terraces on ridge crests and hilltops. The tread or “flat” area is 10 to several hundred meters wide and long and slopes from 1 to 5° parallel to the ridge crests. Terrace scarps may be from 1 to 75 m high. Terraces are cut into all bedrock types and are best developed on closely jointed, fine-grained bedrock. The scarps and treads are covered with frost-rived rubble 1 to 2 m thick. The rubble on treads is perennially frozen at a depth of 1 to 2 m or less on sharp but inactive terraces in Alaska.Cryoplanation terraces exist in many parts of the world in present or past periglacial environments. They occur chiefly in nonglaciated regions and near the general altitude of snowline. Cryoplanation terraces form by scarp retreat as the result of nivation. Surficial debris is removed across the terrace tread by mass-wasting. Terrace morphology depends mainly upon climate, bedrock type, and terrace orientation.No climatic data are available from active terraces. Indirect evidence indicates that climatic requirements include low snowfall and cold summer temperatures. Shallow permafrost is necessary to provide moisture and a base for mass movement as well as a base for nivation.Hundreds of sharp but inactive terraces occur in some areas in Alaska where the summer temperature is colder than 10°C. When these terraces were active, temperatures were colder. Recent work in Alaska indicates that terraces were active in some areas when the mean July temperature was about 4°C. The mean annual air temperature probably was in the neighborhood of -12°C or colder.

Les terrasses de cryoplanation dans le nord du Yukon : distribution, genèse et âge

“…En effet, alors que d'après Reger et Péwé (1976) la surface des terrasses a toujours la pente nécessaire à l'évacuation du matériel érodé, celles situées sur le versant de la montagne Tsi-it-toh-Choh sont horizontales et, de plus, bordées d'un bourrelet ( fig. 7).…”

Réseaux reliques de polygones de tourbe, moyenne et basse Côte-Nord du Saint-Laurent, Québec

“…Parmi ces dernières, certaines comme les palses tourbeuses sont souvent entièrement détruites lors de la disparition du pergélisol et deviennent ainsi inutilisables pour reconstituer les paléoclimats froids ou les paléotempératures du sol. De l'avis général, les indicateurs les plus sûrs demeurent les pingos (FLEMAL, 1976), les réseaux polygonaux à fentes de glace (BLACK, 1976b, SVENSSON, 1978, les terrasses de cryoplanation (REGER et PÉWÉ, 1976) et les formes d'éjection rocheuse (DIONNE, 1983). D'autres formes et phénomènes périglaciaires comme les géli-sols avec triage (GOLDTHWAIT, 1976) la gélifluxion (BENEDICT, 1976), les glaciers rocheux (WHITE, 1976) les palses minérales (PAYETTE et SEGUIN, 1979;LAGAREC, 1982) Les connaissances sur les variations climatiques au Québec et en particulier au Québec méridional, depuis la dernière glaciation, demeurent fort limitées 1 .…”

The Age and Origin of the Little Diomede Island Upland Surface

“…Cryoplanation terraces are widespread on the uplands in nonglaciated portions of central Alaska, including the western tip of Seward Peninsula (Reger and Péwé, 1976). Reger and Péwé (1976) suggest formation of the surfaces is largely due to scarp retreat as the result of nivation.…”

“…The upland may be classified as a cryoplanation surface: a land surface reduced to low relief by processes associated with intensive frost action, supplemented by the actions of running water, moving ice, and other agents (Bates and Jackson, 1983;Allaby and Allaby, 1991). Reger and Péwé (1976) classify cryoplanation surfaces as bedrock steps or terraces on ridge crests and hilltops, chiefly occurring in nonglaciated areas near the general altitude of the snowline. Cryoplanation terraces are widespread on the uplands in nonglaciated portions of central Alaska, including the western tip of Seward Peninsula (Reger and Péwé, 1976).…”

“…Reger and Péwé (1976) classify cryoplanation surfaces as bedrock steps or terraces on ridge crests and hilltops, chiefly occurring in nonglaciated areas near the general altitude of the snowline. Cryoplanation terraces are widespread on the uplands in nonglaciated portions of central Alaska, including the western tip of Seward Peninsula (Reger and Péwé, 1976). Reger and Péwé (1976) suggest formation of the surfaces is largely due to scarp retreat as the result of nivation.…”