Response to ‘Discussion: “Streamlined erosional residuals and drumlins in central British Columbia, Canada”’

Response to ‘Discussion: “Streamlined erosional residuals and drumlins in central British Columbia, Canada”’

Geomorphology 209 (2014) 151–153 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Discuss...

168KB Sizes 0 Downloads 4 Views

Geomorphology 209 (2014) 151–153

Contents lists available at ScienceDirect

Geomorphology journal homepage: www.elsevier.com/locate/geomorph

Discussion

Response to ‘Discussion: “Streamlined erosional residuals and drumlins in central British Columbia, Canada”’ J. Donald McClenagan ⁎ Telkwa, B.C. V0J 2X1, Canada

a r t i c l e

i n f o

Article history: Received 2 December 2013 Received in revised form 8 December 2013 Accepted 9 December 2013 Available online 16 December 2013 Keywords: Streamlined erosional residual Anastomosing channel Megaflood Landform shape

a b s t r a c t A response is given to ‘Discussion: “Streamlined erosional residuals and drumlins in central British Columbia, Canada”’. Emphasis is made that the main purpose of the paper under discussion is to present the recognition of a distinctive landscape pattern in central British Columbia that appears to be an immense anastomosing channel network. A channel network of the magnitude described requires a large magnitude of flowing water to form it. Thus, that recognizable landscape pattern and associated upland landform shapes can be explained as products of water erosion. Such landscape patterns are observed being formed by water today. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The stated hypothesis of the article under discussion is: ‘…study area uplands defined by closed contours are erosional residuals that have attained their main shape, when viewed in the horizontal plane, from streamlining fluvial flow…’ (McClenagan, 2013). The primary intent of my article, then, is to propose a mechanism for the development of the landscape-scale features (uplands and lowlands) in central British Columbia, Canada. Stumpf et al. (2013) in their discussion addressed mainly, though not entirely, the topic of drumlins and other similarly scaled streamlined forms (hereafter termed drumlins) occurring in the study area. Drumlins are not landscape-scale and are not included in the stated hypothesis. Although drumlins are discussed in my article, the main thrust of the article is the presentation of evidence and arguments that concern the development of the upland–lowland landscape assemblage, as stated in the hypothesis. Stumpf et al. (2013) argue that I ‘failed to provide an adequate analysis of existing … information … necessary for a reader to thoroughly evaluate the merits of the hypothesis.’ I maintain that adequate evidence is present in the article to, at the least, generate a valid consideration of the idea that the landscape was formed by a catastrophic flood or floods. The reader is asked to consider the remarkable similarity of the study area landscape to other anastomosing channel networks

⁎ Tel.: +1 250 846 5004. E-mail address: fi[email protected] 0169-555X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.geomorph.2013.12.007

recognized as flood-generated landscapes, presented examples including the Channeled Scablands of central Washington State and outburst flood channels at the margins of the Laurentide Ice Sheet. Additionally, numerical shape analysis of study area uplands substantially agrees with similar numerical analyses of erosional residuals in known flood channels. 2. Present geological model for plateau region of central British Columbia Stumpf et al. (2013) charged that I did not adequately present the current understanding of (i) drumlin formation and (ii) landscape evolution in the region. I agree with the first point. As stated in the Introduction above, the hypothesis and main thrust of the paper pertains to landscape-scale features of the plateau region, not to drumlins. Indeed, there is an extensive body of work concerned with drumlin occurrence and origins that is not covered in the article. A survey of existing ideas concerning drumlin origins and occurrence was not the intent of the article. I disagree with the second point. The article under discussion proposes an explanation for the measurable shapes of uplands and the observed pattern of uplands and interconnecting lowlands in the study area. The plan view shapes of these uplands and the related interconnecting channel system is presented as the development of land surfaces presenting the least resistance to fluvial flow, thus providing an explanation for the shape of the landscape. I do not know of another study that proposes a mechanism explaining why the study area landscape has its current form.

152

J.D. McClenagan / Geomorphology 209 (2014) 151–153

Stumpf et al. (2013) stated: ‘To treat this body of work [concerning drumlin occurrence and origins in the study area] in such a cavalier manner and then proceed directly into description of drumlins from other parts of glaciated North America and northern Europe is disingenuous to readers….’ This statement is in error. The article in question does not discuss drumlins in other parts of glaciated North America and northern Europe. It does, however, discuss landscapescale streamlined erosional residuals in those geographic areas. These erosional residuals are not drumlins nor are they presented as such in the article.

referred to as ‘drumlins’ in the recent article. My intent at using somewhat imprecise terminology was to avoid a digression of questionable worth and unquestionable length in the article. In defense, there is a section on terminology in my thesis that surveys the various, and sometimes ambiguous, terms used to describe what I call drumlins in this recent article. Finally in this section, Stumpf et al. (2013) suggested that drumlins and other similarly scaled streamlined forms may have their directions influenced by bedrock structure, bedrock lithologies, and bedding orientations. I agree with that suggestion, and I do not believe I express any statement contrary to that suggestion in my paper.

3. Glacial dispersal Stumpf et al. (2013) have referenced research that documents mineral dispersal trains aligned with orientation of drumlins. Stumpf et al. also pointed out that most drumlins in the area are composed of a ‘massive diamicton interpreted as till’. The primary purpose of the paper is to discuss landscape geomorphology, not drumlins. I will nonetheless attempt to respond to these points. Stumpf et al. (2013) argued that, because dispersal train orientations show relationship to drumlin orientations, then the drumlins must have been formed by glaciers. They appear to base this conclusion on the assumption that dispersal trains are a result of glacial actions. Although the assumption that most geomorphic and depositional features in central British Columbia are glacial in origin may be widely accepted, it is that very assumption that the evidence of landscape-scale fluvial erosion calls into question. If dispersal trains were a response to fluvial activity instead of glacial activity, then the very reasoning used by Stumpf et al. (2013) to argue for a glacial origin of drumlins could be used to argue a fluvial origin for drumlins. Diamicton can be produced by water-generated debris flows (e.g., Jackson et al., 1987; Eyles and Kocsis, 1988). Even if dispersal trains were deposited by ice movement, this does not prove that associated drumlins were formed by ice. Catastrophic floods could have flowed in the same direction as the ice movement. 4. Cross-cutting relationships and drumlin morphology Stumpf et al. (2013) maintained that field evidence indicates a complex ice-flow history, which I did not discuss. It was not the intent of my paper to discuss ice-flow history in the study area. I refer to the last sentence of the article, which states: ‘Drumlins likely formed as the anastomosing network developed; divergent drumlin orientations probably reflect either late flow or separate drumlin-forming events that did not appreciably alter the anastomosing system…’ (McClenagan, 2013, p. 50). This statement allows, at least to some degree, the observations of divergent flow directions referred to by Stumpf et al. (2013). Stumpf et al. (2013) stated ‘we question whether all the features identified by McClenagan (2013) were drumlins.’ Indeed, all the features I identified are not drumlins as is stated plainly in the introduction to the article (please see Section 1.2 of the article). The paper deals primarily with streamlined erosional residuals (SER) and secondarily with drumlins. Drumlins in the study area average about 0.5 km in length with a standard deviation of about 0.3 km (McClenagan, 2005). Although the study area closed contours interpreted as SER can be at the same scale as drumlins, the detected closed contours can range up to about 100 km in length. The shapes of the uplands interpreted as SER in the study area are like streamlined islands and braid bars found in river or flood channels (Figs. 7–12 in McClenagan, 2013). Drumlins are not shaped like most SER (Fig. 15 in McClenagan, 2013). Stumpf et al. (2013) have some grounds for their objection to the use of the term drumlin for all drumlin-scale features discussed in the article. I agree that some features that have been referred to as rock drumlins, crag-and-tails, and flutings were more than likely mapped as ‘streamlined forms’ in my thesis (McClenagan, 2005) and subsequently

5. Glacial erosion model In Section 5 of their discussion, Stumpf et al. (2013) listed other studies in which the length-to-width ratios of drumlins have been analyzed. They stated that ‘Other researchers…have conducted similar analyses of drumlin morphometry in areas of low and moderate relief and found similar length-to-width ratios.…’ They implied that I, too, was analyzing the length-to-width ratios of drumlins. Apparently, Stumpf et al. (2013) misunderstood one of the main methods described in the paper. The length-to-width ratios that I measured were not of drumlins but of erosional residuals (see Sections 2.2, 3.3, 3.5, and Table 1 in McClenagan, 2013). I question whether studies of drumlin morphometry should be used to refute a study of erosional residual morphometry. The two geomorphic features are entirely different. 6. Megaflood or megafloods? Stumpf et al. (2013) stated: ‘The erosion…would produce large amounts of sediment that should be present down flow—however, none has been reported.’ I agree with the first part of the statement, that large amounts of sediment should be present down flow. However, I am not certain that thick sedimentary sequences have not been reported. Given the potential magnitude of the hypothesized flood(s), the location of such deposits could be hundreds of kilometers from the study area. I have not conducted a literature review or field work to research the occurrence of such deposits; however, I have many times observed and remarked the sequences of sorted gravels and sands many meters thick in the Fraser canyon in south central British Columbia. I would conjecture that the Fraser River at its current size could not lay down such thick sequences. Stumpf et al. (2013) contended that deposits of fluvial sediments were not found at upper elevation sites in the study area. I would not expect to find any. If the streamlined features at those locations were formed by water, the velocities and magnitudes of those floods would more than likely completely remove the material not deposit it. Stumpf et al. (2013) also questioned whether there is any valid explanation for a source of the megaflood or megafloods. They stated that ‘the source for the proposed flooding…remains enigmatic.’ I disagree with the implication that a source for the flood must be identified before the evidence, as presented in the paper, can be accepted as indication of a megaflood(s). I propose an analogy. If a field geologist finds a thick deposit of well-sorted gravel, he or she would almost certainly infer that the gravels were deposited by water. There are many examples of gravel bars known to be water deposits. Furthermore, we can physically explain why sorting takes place. I argue that the landscape pattern in the study area is analogous to a well-sorted gravel deposit. The landscape pattern is remarkably similar in appearance to a fluvially produced anastomosing channel network and that the uplands are analytically shown to be shaped like erosional residuals in flood or river channels. Although the identification of a source or sources for the proposed megaflood(s) is certainly desirable, the lack of that identification

J.D. McClenagan / Geomorphology 209 (2014) 151–153

does not invalidate the recognition of the study area landscape as a fluvial feature. As a side note, I caution against using estimates of the depth of flow based on current upland–lowland elevation differences. If the existing valley system was eroded by an immense megaflood(s), then the valley floor deepened during that event and the elevation difference increased.

7. Summary The main purpose of the paper under discussion is to present the recognition of a landscape pattern in central British Columbia that appears to be an immense anastomosing channel network. A channel network of the magnitude described requires a large magnitude of flowing water to form it. Thus, that recognizable landscape pattern and associated upland landform shapes can be explained

153

as products of water erosion. Such landscape patterns are observed being formed by water today. I thank Stumpf et al. (2013) for taking the time to discuss this paper. References Eyles, N., Kocsis, S., 1988. Sedimentology and clast fabric of subaerial debris flow facies in a glacially-influenced alluvial fan. Sediment. Geol. 59, 15–28. Jackson, L.E., Kostaschuk, R.A., MacDonald, G.M., 1987. Identification of Debris Flow Hazard on Alluvial Fans in the Canadian Rocky Mountains. Department of Geography, University of British Columbia (34 pp.). McClenagan, J.D., 2005. The Occurrence and Origins of Streamlined Forms in Central British Columbia. (Ph.D. thesis) University of Victoria, BC (371 pp.). McClenagan, J.D., 2013. Streamlined erosional residuals and drumlins in central British Columbia, Canada. Geomorphology 189, 41–54. Stumpf, A.J., Ferbey, T., Plouffe, A., Clague, J.J., Ward, B.C., Paulen, R.C., Bush, A.B.G., 2013. Discussion: “Streamlined erosional residuals and drumlins in central British Columbia, Canada” by J. Donald McClenagan. Geomorphology (ISSN: 0169-555X) 189, 41–54. http://dx.doi.org/10.1016/j.geomorph.2013.10.019 (Geomorphology, Available online 30 October 2013).