Time‐transgressive cryoplanation terrace development through nivation‐driven scarp retreat

Abstract

Cryoplanation terraces are elevated bedrock features resembling giant staircases, with alternating steep scarps and shallow sloping treads. These landscape‐scale features have long been associated with periglacial environments, but the processes involved in their formation remain vaguely specified and contentious. The two leading hypotheses for the formation of cryoplanation terraces are centered on: (1) geologic structure; and (2) nivation‐driven scarp retreat. The nivation‐formation hypothesis invokes scarp retreat under erosion processes associated with late‐lying snowbanks. To test whether cryoplanation terraces develop through scarp retreat, six relative weathering indices (fracture counts, Cailleux roundness, Cailleux flatness, Krumbein sphericity, rebound, and weathering rind thickness) were measured across well‐developed terraces in unglaciated eastern Beringia (central and western Alaska) at Mt Fairplay, Eagle Summit, and Skookum Pass. Statistically significant differences in relative weathering indices detected through chi‐square and multiple‐comparison procedures indicate that material is less weathered closer to scarps, i.e. that these areas were more recently exposed than those distant from the scarp. Based on these findings, a refined model of time‐transgressive cryoplanation terrace development through nivation‐driven scarp retreat is proposed. This new qualitative model addresses the removal of weathered material from terrace treads down side slopes through piping and gravity‐driven mass‐wasting processes. © 2019 John Wiley & Sons, Ltd.

Cosmogenic10Be and36Cl geochronology of cryoplanation terraces in the Alaskan Yukon-Tanana Upland

“…Owing to the similarity between the rates reported here and those from process-oriented nivation studies elsewhere, as well as ages synchronous with local glacial advances, we conclude that the nivation process suite is likely to have produced the CTs studied in this work during Quaternary cold intervals, thereby exposing these elevated boulders. The results presented here are also consistent with previous work by Nyland and Nelson (2019) that proposed a refined model for CT formation in eastern Beringia based on nivation after examining spatial trends in a variety of relative weathering indices across all terrace treads present at these two study sites.…”

“…Cosmogenic 10 Be and 36 Cl geochronology of cryoplanation including, Kňažková et al (2018) who reported scarp retreat under nivation processes on James Ross Island, Antarctica, of 0.77 ± 0.12 mm/yr (0.077 ± 0.012 cm/yr), and Matthews et al (2019) who reported maximum rates of ∼0.1 mm/yr from Svartkampan, Jotunheimen, Norway. Indirect evidence also supports the nivation hypothesis for CT formation across eastern Beringia (Reger, 1975;Reger and Péwé 1976;Nelson, 1989Nelson, , 1998Nelson and Nyland, 2017;Nyland and Nelson 2019). Owing to the similarity between the rates reported here and those from processoriented nivation studies elsewhere, as well as ages synchronous with local glacial advances, we conclude that the nivation process suite is likely to have produced the CTs studied in this work during Quaternary cold intervals, thereby exposing these elevated boulders.…”

“…Indirect evidence also supports the nivation hypothesis for CT formation across eastern Beringia (Reger, 1975; Reger and Péwé 1976; Nelson, 1989, 1998; Nelson and Nyland, 2017; Nyland and Nelson 2019). Owing to the similarity between the rates reported here and those from process-oriented nivation studies elsewhere, as well as ages synchronous with local glacial advances, we conclude that the nivation process suite is likely to have produced the CTs studied in this work during Quaternary cold intervals, thereby exposing these elevated boulders.…”

“…Fairplay using the 2 m resolution ArcticDEM (Porter, et al, 2018) revealed evidence of well-defined, steep landslide scarps. Although it is not possible to definitively associate this anomalously high scarp with landslide processes, it is possible that nivation modified a preexisting structural morphology, as in the model for CT formation proposed by Nyland and Nelson (2019).…”

Inherited periglacial geomorphology of a basalt hill in the Sudetes, Central Europe: Insights from LiDAR‐aided landform mapping

“…These less inclined sections, in turn, lose clarity and eventually merge with the surrounding slope surface. Jointing characteristics and cliff/slope morphology match the intact strength of basalt in the Muchów Hills in that tall vertical cliffs have developed in massive and mechanically stronger rocks (mean R-values from 53.8 ± 2.06 to 58.8 ± 3.25 SD), whereas lower R-values (from 47.3 ± 2.46 to 55.6 ± 3.26 SD) were recorded within less conspicuous rock steps supported by thinner columns.Low-inclination surfaces downslope from the bedrock cliffs, comparable to cryoplanation terraces,1,[34][35][36] are rare. The best examples of such benches can be found on the south-facing slopes, in the west-central part(Figure 2E).…”