Comments on: Discriminating viscous creep features (rock glaciers) in mountain permafrost from debris-covered glaciers – a commented test at the Gruben and Yerba Loca sites, Swiss Alps and Chilean Andes

Stephan Harrison

This is an interesting and thought-provoking article and I am glad to be able to comment on it.  In my view, it should be published after some minor additions.  I absolutely do not believe in everything written in the paper; but I do not think that this matters so much.  I am a strong believer in airing competing (and sometimes contradictory) views in science and this is a case in point.

In that light I have discussed some issues below which could serve as alternative discussion.  These views could be incorporated in a revised version, but I understand if they are not.  The first author and I have had several lengthy email discussions about this and I have synthesised my views below.   At times, it has seemed that our respective positions have been far apart.  I believe that rock glaciers may display equifinality, with many being of 'periglacial origin, but some also containing large amounts of glacial ice, and therefore be partly derived from glaciers.  I am therefore glad to see that maybe our views are actually converging.

For instance, on Lines 25-28 the paper states:

“Where debris-covered surface ice is, or has recently been, in contact with thermally-controlled subsurface ice in permafrost, complex conditions and interactions can develop morphologies beyond simple “either-or”-type landform classification. In such cases, remains of buried surface ice mostly tend to be smaller than the lower size limit of “glaciers” as applied in glacier inventories, and to be far thinner than the permafrost in which they are embedded”.

I would generally agree with this statement because it recognises the complexity of some of these landforms. It is clear to me that substantial thicknesses of buried ice can exist in rock glaciers (between 25 and 50 m thick on one we are working on in the Himalaya) and I cannot see how this can be explained in the ‘permafrost model’.

I agree that viscous flow (and our understanding of this) is an important issue if we want to be able to understand rock glaciers and identify them.  I would also agree that when we can identify this in a rock-ice landform then we can say that we have some rock glacier characteristics.  But I can see several problems with this.  I will mention two main ones.

First, if we say that rock glaciers must exhibit ONLY viscous flow, then there are lots of hybrid features that we would have to call something else.  For instance, as you know some features that we would call rock glaciers may have elements of viscous flow via an ice-cemented layer, but also might also contain more massive ice which might not behave like this at all, as my previous comment indicates.  As WH and I talked about in a previous conversation, I believe that this is because there must be a continuum of landforms, and that the definition of “rock glacier” must account for this.  

Second, when undertaking inventories, how can we know that the features we are calling rock glaciers have all undergone viscous flow?  OK, there are flow structures on the surfaces, but until you do detailed field analysis or remote sensing on the feature you cannot unequivocally say.  When there are thousands of rock glaciers in a region (as in the arid Andes or Himalaya) then this is a real problem, and we have to make some extrapolation from small samples.  One alternative is to call these I-DL (which is what Harrison et al. (2008) and Jarman et al (2013) did a few years ago) and this would be one way out of this conundrum.  Both of these papers should be cited in this article.  

So we are left with us having to accept a degree of coarse-graining (as discussed in Harrison (1999 and 2001)  and also the problem of equifinality.  I have stood in front of the Morennij rock glacier in Kazakhstan and seen a 'turf-wave' in front of it, which suggested to me that the feature (or part of it) was undergoing some basal sliding.  I have also climbed up the front where this is substantial (>5m) of massive ice exposed as the frontal talus material was disturbed, , so I would argue that at least part of this has some glacial origin.  Is this still a rock glacier? 

You have essentially agreed with this point in Lines 108-110 when you say “Where mean annual air temperatures are, or have until recently been, below zero centigrade, various types of surface ice - especially cold perennial snow fields or ice patches, glacierets and mostly small, cold to polythermal glaciers - can be in contact with thermally controlled, creeping ice-rich mountain permafrost, contributing debris and in cases remains of buried ice to deeply frozen materials”.

 You state this again (line 117)

“Contacts and interactions between glaciers and rock glaciers can give rise to a diverse range of landforms, exhibiting a wide spectrum of characteristics” and also in line 122

“Such complex and highly variable landforms can mostly not be attributed in a straightforward “either-or” scheme to the terms “rock glacier” or “debris-covered glacier” but constitute what could better be called “complex contact zones of (viscous creep in ice-rich) permafrost with remains of buried surface ice”. 

As mentioned above, Jarman et al 2013 used the term ice-debris landform (I-DL) for such features and this usage followed from Harrison et al. (2008) and identified these as “a mappable depositional feature owing its present form substantially to forward (downslope) movement rheologically facilitated by deformation of interstitial, incorporated or underlying ice” (Jarman et al. 132).  Of course, we were discussing relict Plesitocene features with no ice content, and where it is therefore difficult to assess the nature of the underlying nature of the constituent ice.  However, I do think that we are facing a continuum of various features with various amounts of ice of varied origin, and this supports your quote on line 122.

In essence, any classification scheme has practical problems.  All rock glaciers are different from each other, but all have some characteristics that are the same for the group.  These characteristics allow us to map them and describe them without the enormous logistical problem of having to drill boreholes within each one.  And even drilling only tells us about one small part of the feature.  I would argue that there may be lots of ways in which such features have developed (and may develop in future), and this is the equifinality issue.  A true reductionist would say that equifinality is just a problem of graining and classification.  And I would agree, but no classification system could EVER account for the uniqueness of ALL features we call rock glaciers. 

So if we don't have some emergent view, then we end up by having no classification system, no laws, and only descriptions!  And I am sure that we would all agree that this would be a very poor outcome.  Hence, I am a pragmatist.  I believe that features we call rock glaciers can evolve in different ways, and that we can't possibly know how each one evolves.  What we strive for is a way to understand process and development, and thus how we classify.  And we need to understand these features in how they contribute to local hydrology.

I am glad that our respective views appear to be converging.

Several of the below should be cited in a revised version:

Knight, J., 2019. A new model of rock glacier dynamics. Geomorphology, 340, pp.153-159.

Jones DB, Harrison S, Anderson K . 2019.  Mountain glacier to rock glacier transition. Global  and Planetary Change. http:// doi: 10.1016/j.gloplacha.2019.102999

Knight J, Harrison S, Jones DB.  2018.  Rock glaciers and the geomorphological evolution of deglacierizing mountain. Geomorphology, 324, 14-24.

Jarman D, Wilson P and Harrison S.  2013. Are there any relict rock glaciers in the British mountains?   Journal  of Quaternary Science. DOI: 10.1002/jqs.2574

Harrison S, Whalley B and Anderson E.  2008.  Rock glaciers in the British Isles: implications for Late Pleistocene mountain geomorphology and palaeoclimate.  Journal of Quaternary Science, Vol. 23, 287-304.

Harrison S.   2001. On reductionism and emergence in geomorphology.  Transactions of the Institute of British Geographers, Vol. 26(3), 327-339.          

Harrison S.  1999. The problem with landscape: some philosophical and practical questions. Geography, Vol. 84(4), 355-363. 

Comment on “Discriminating viscous creep features (rock glaciers) in mountain permafrost from debris-covered glaciers – a commented test at the Gruben and Yerba Loca sites, Swiss Alps and Chilean Andes”

Brian Whalley Department of Geography, University of Sheffield, Sheffield, UK

Fethi Azizi,  School of Engineering, Computing and Mathematics, University of Plymouth, Plymouth, UK

Correspondence to: W. Brian Whalley (b.whalley@sheffield.ac.uk)

Haeberli et al. (2023) compare two sites to discriminate between rock glaciers and debris-covered glaciers. They also indicate problems in accepting a ‘glacigenic’ origin for rock glacier (RG) origins compared with a permafrost (‘cryogenic’) origin. ‘Science is the empirical method of knowing based on systematic observation and theorization’ (Preston et al., 2023); observations and theory may come from several sources. Famous debates, such as the ‘big-bang’, associated with certain scientists (Kragh, 2013), can be of long-standing. An earth science example is the ‘Channelled scabland’ (Waltham, 2010). 

The distinguished glaciologist Louis Lliboutry (1990), commented, ‘I do not wish to enter into a public controversy with W. Haeberli about the origin of rock glaciers; he has always been deaf to my arguments. Nevertheless, the readers of his passionate assertions (Haeberli, 1989) must be aware that he intentionally omits to quote my detailed observations in the dry Andes (Lliboutry, 1955, 1965, 1986)’. Specifically, Lliboutry elaborates, ‘The concept that many rock glaciers come from old glaciers, or from "buried glacierets" (i.e. debris-rich glacierets, entirely covered at the end of the ablation season, and not nourished by ice every year) is not at all "purely speculative", as he [Haeberli, 1989] says’. Some 30 years later, Lliboutry’s observations and ours are consistent with a glacigenic viewpoint.

In presenting actual, not speculative, observations and theory, we outline, 1. continuum mechanics and material properties (Whalley and Azizi, 1994), 2. Observations at Gruben RG (Whalley, 2020) and 3. the Yerba Loca and elsewhere in the ‘dry Andes’ (Whalley, 2023a). As in Whalley (2021b) we use a decimal [latitude,longitude] geolocation to identify features in open formats such as Google Earth^TM (GE) for ground truth investigations (Whalley, 2023b).

Haeberli et al.’s (2023) definition of RG uses Wahrhaftig and Cox (1959). Although the latter discuss the ‘Glen’ flow law, they use a single parameter (viscous) model to explain their kinematic observations of slow flow. They also note surface trial pits showing ‘interstitial’ ice.  This is hardly surprising in an area of continuous permafrost, as also in the Wrangell Mountains (Capps, 1910), ‘The valleys are still on the very border line of glacial conditions, and in fact many of them still have small glaciers at their heads’ for example Jumbo RG [61.5223,-142.8460] which terminates as two snouts [61.5150,-142.8584] and [61.5130,-142.8578], sequential down-valley.

All geomorphological materials (rock mass and fragments, soil, water, ice, lava) in any combinations must move according to some ‘flow criteria’; geometric applied stresses such as thickness and surface slope. Geotechnical Engineering analyses, involving the strength properties of granular and fine grained materials, partially or fully saturated, is routinely used to model and predict slope failures (Azizi, 2007). Where glacier ice is involved, creep modelling of long-term temperature-dependent plastic deformations, can be used - the stress-strain rate-temperature Glen-Steinemann ‘flow law’ with an exponent (n) ranging between 2 and 4 is a typical example.

The term ‘viscous creep’, even when associated with low RG velocities ‘which reflect cumulative deformation through slow viscous creep of perennially frozen talus/debris rich in ice’ Haeberli et al. (2023), does not uniquely define the appropriate constitutive equation. Azizi (2007), Azizi and Whalley (1996) and Whalley and Azizi (1994) discuss the rheology showing, as is well-known, that thin glaciers (<30m) at low surface gradients (<10˚) creep with annual surface velocities <1m/a. That is, they are debris-covered glaciers. Conversely, ‘creeping permafrost’ as frozen rockfill will not creep (Whalley, 1974; Whalley and Azizi, 2003). Substantial (>20m) ice thicknesses are required for observable creep to occur, even on steep hillsides (Whalley and Azizi, 2003), particularly at ‘low’ temperatures. Note that scree (talus) slopes near valley floor RG do not show creep/flow characteristics as can be seen in the locations referred to in this note and elsewhere.

Gruben RG [46.1718,7.9624] and Grubengletscher [46.1673,7.9675] were mapped as a landsystem of debris-buried glacier ice (Whalley, 1974, 1979). Haeberli et al. (2023, Figure 1) indicate Grubengletscher as a ‘cold, debris-covered glacier tongue’ with no evidence for its temperature regime. Whalley (2020, and supplementary material) showed that the LIA glacier ice in the Gruben basin was clearly mapped as such in 1850, 1876. A subsequent series of Swisstopo (Landeskarte der Schweiz) maps show a debris-free Little Ice Age glacier that became debris-covered, especially the right-hand portion, i.e. Gruben RG (Whalley, 2020).

Haeberli (1985, p. 120-1) interpreted Gruben RG as a permafrost body because of the low surface velocity (<1m/a); a kinematic explanation. Landsystem mapping (Whalley, 2020, Fig. 5) revealed a glacier ice core in several meltwater pools, especially post 2000. The glacier presence can be explained by low slope angle, a thin glacier core and a glacier flow law, i.e. a dynamic explanation from continuum mechanics (Whalley, 2020, Whalley and Azizi, 1994, 2003). These observations confirm that rock glaciers are indeed debris-covered LIA glaciers. Continuity of ice mass, from observed glacier to burial below insulating debris, now shows  meltwater pond ‘windows’ through the debris cover (Whalley, 2020 Figs 3, 4, 11). The ice exposed on the steep ice cliffs in these melt pools is large-crystal glacier ice with little entrained debris. Debris slumping may subsequently fill in these melt pools, sometimes they are preserved over time, as seen in GE.    

A melt pool is seen on the Galena Creek RG [44.642,-109.791@2020] at [44.63987,-109.79140] that has widened and moved downstream since 2009 (Whalley, 2023b). This site has also been used to ground truth radar imaging of the glacier body (Petersen et al., 2020). These observations, together with an extracted ice core (Whalley, 2023b), show the glacier ice continuity in this RG, confirming the observations of Potter (1972) but negating the criticisms of Barsch (1987, 1996). Such well-developed, longitudinally-distributed melt pools are also seen on the ‘transition’ of glacier to debris covered glacier to glacier ice cored RG [-30.2414,-69.8541] (Whalley, 2023a). A similar sequence of surface melt pools is seen at [-33.2150,-70.2461] at the Yerba Loca site. GE inspection shows several melt pools developing in size, number and downstream extent over some 20 years. These steep-sided pools are glacier melt phenomena and show the continuum presence of glacier ice, whether or not this is at sub-pressure-melting point (Lliboutry, 1990).

Steep frontal RG slopes are not ‘over steepened’, they are at the appropriate resting angle of the granular material. They do indicate the proximity of a glacier ice core below surface debris  as shown at Nautardalur RG, Iceland [65.4895,-18.3714] (Whalley, 2021a). The proximity of glacier ice cores explains, via  traditional glacier physics of compressive deviatoric stresses of an incompressible body (Hooke, 2019) that extending flow (as at Nautardalur) gives lateral moraine ridges providing ‘organization’ to ‘chaotic’ boulder distribution on the debris-covered glacier. A compressive flow regime tends to bring surface debris to the front of the RG, impeding flow but producing lava-like ridges as medium-discontinuous ‘roll waves’ in the frictional debris layer above the glacier ice continuum. Both extensive flow, producing lateral moraines [44.5581,6.8634] and compressing flown regime with a ‘ridge and furrow’ snout [44.5621,6.8634], are seen at Marinet RG (Whalley and Palmer, 1998).  

Conclusions

The observations provided above are freely available and can be confirmed in GE for the present and past interpretation and allow future predictions. These observations show that rock glaciers can indeed have glacier ice cores. Repeated insistence of the ‘permafrost model’ (affirming the consequent) does not make the ‘cryogenic’ interpretation correct in logic or glaciology.

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