Introduction
conducted field research to find evidence of ancient upland glaciation “in the absence or near-absence of icecontact indicators”, and as an example they presented a case study mainly from the Late Paleozoic palaeoequatorial Cutler Formation (Colorado, United States). This is a good aim, because upland areas are subject to much higher rates of erosion than lowland or marine environments, the latter which more often exhibit deposition. Even today there are glaciers close to or even at the equator, on e.g., Mount Kenya ().
As describe, the problems of identifying ancient upland glaciation are substantial. A large part of the paper is used to try to explain how fluvial deposits can be interpreted as glaciofluvial. The work is diligent, but there is no definite answer to the origin based on the appearance of the deposits themselves.
Discussion—General
Most of the pictures presented by of geologic features, which would be the more decisive ones for influence from cold weather, are from other areas than the Cutler Formation. Features interpreted to be pseudomorphs from ice-crystals may be chemical, from e.g., salt mineralization, as mention. However, occasional snowfalls may take place on lower land surfaces close to the equator (), and the ice-crystal pseudomorphs pictured appear to be perfectly similar to recent ones (; ). However, none of these ice-crystals are from the Cutler Formation.
“Frozen ground phenomena” described and pictured are not from the Cutler Formation, but from the Pennsylvanian Fountain Formation, Colorado. refer to a paper by , which describe these fractures and polygons to “range from 3 cm to 55 cm to 13 cm–>220 cm in width and depth, respectively”. The patterned ground documented appears to display a network of squeezed sediments (; , their fig. 5), like what may be induced by gravity from soft sediment tectonics (e.g., ), but more seldom from freeze-thaw cycles. Freeze and thaw polygons may be straighter and more vertical, but not displaying an underground network of sediments. The size and appearance of the polygons described by are not much similar to polygons which are made by freeze-thaw-cycles. The latter commonly are more regular and in sizes often many times larger (), while the interiors documented by are displayed as an irregular “condensed” network.
Slices and balls of soft non-cohesive sediment, i.e., rip-up clasts, are incorporated in the deposits, and are interpreted to have been in a frozen condition. The only reference by to sediments “which appear analogous to rip-ups of frozen bank material” is a paper by . wrote “It is possible that the sand megaclasts were transported to the site in a frozen condition.” Any material inside and especially at the bottom of a glacier is heavily molded, and clasts are quickly rounded. Any frozen sediment would be more prone to such molding. At the other end, soft non-cohesive sediments are commonly transported with mass flows (e.g., ; ), which takes place regularly and so often that such sediments actually may be used as an indication of sediment gravity flows. In conclusion, soft non-cohesive sediments, either mud balls covered with sand or gravel giving them an appearance of sandstone (; ), or balls or slices of non-cohesive sediments, are common constituents of sediment gravity flows and flash flood sediments (; ; ). The evidence from incorporated soft sediments therefore is better explained by sediment gravity flows than glaciation.
Discussion—Surface microtextures
A major shortcoming in the paper by is their interpretation of surface microtextures on quartz grains. Papers and books by e.g., , , and , even if different methods are used, show how and when surface microtextures originate and change during transport.
wrote “More recent work has argued that only large-scale fractures that cover at least one-quarter of the grain surface can be considered glaciogenic, as smallerscale fractures can be produced in a wide variety of environments ().” This statement is in part almost opposite to the quoted work of . Fractures by themselves indicate almost nothing. They commonly originate during release from bedrock or in any high-stress environment (; ). It is the combination and appearance of surface microtextures which is important. The conclusion in the paper by is: “1 A glacigenic grain typically exhibits largescale fractures (F1) and irregular abrasion (A1),” Later work has further specified this combination of surface microtextures, i.e., fresh fractures that are irregularly abraded (e.g., ).
refer to a paper by to indicate that surface microtextures can be preserved during long fluvial transport. However, the sand grains pictured by , their fig. 3 display different fractures, or actually fractured grains, and not any abrasion, neither regular abrasion from fluvial transport nor irregular abrasion from glacial grinding (e.g., ; ; ). Their pictured grains are all similar to grains that have only been fractured, by any high-stress process (e.g., this also occur in strong rivers displaying clasts that are tumbling around) and these grains are also similar to the Late Paleozoic grains pictured by , their fig. 1). documented how surface microtextures are transformed, from the typical glaciogenic irregularly abraded fractures, to regularly abraded grains, from transport by water. Only in a sample transported less than 1,000 m, maybe only c. 100 m, there were no difference compared to tills from the same area, but sand grains that had been transported longer distances were more regularly abraded and therefore also displayed less large scale fractures. also wrote “no systematic study exists to assess quartz-grain microtextural variation due to thickness of ice in alpine settings”, and have missed the documentation in . This paper () also presented a method to avoid operator bias or variance, as was not recognized by .
Conclusion
In conclusion, the paper by , is an attempt to fill in a gap in our knowledge, but it fails to show the evidence that is needed. In particular the section concerning surface microtextures is superficial and faulty, and the presented data do not show evidence of any former glaciation.
Statements
Author contributions
The author confirms being the sole contributor of this work and has approved it for publication.
Conflict of interest
MM was employed by Umea FoU AB.
Publisher’s note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
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Summary
Keywords
diamictite, surface microtextures, patterned ground, sediment gravity flow, upland glaciation, pre-Pleistocene record
Citation
Molén MO (2023) Comment to: Detecting upland glaciation in Earth’s pre-pleistocene record. Front. Earth Sci. 11:1120975. doi: 10.3389/feart.2023.1120975
Received
06 January 2023
Accepted
31 March 2023
Published
12 April 2023
Volume
11 - 2023
Edited by
John L. Isbell, University of Wisconsin–Milwaukee, United States
Reviewed by
Tae-Yoon Park, Korea Polar Research Institute, Republic of Korea
Updates
Copyright
© 2023 Molén.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Mats O. Molén, mats.extra@gmail.com
This article was submitted to Cryospheric Sciences, a section of the journal Frontiers in Earth Science
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.