Saturday, 6 April 2013

More climate clues on the Northern Tablelands

In January last year I did a post called How Cold Was It? Glaciers in New England? that showed evidence of peri-glacial features in the Northern Tablelands of New England, specifically in the area just to the east of Guyra. Bob H, gave me a tip-off for these interesting features which went unnoticed for a long time – including by me. I’d even taken a photograph of a solifluction lobe and not identified its true nature! It is important to know that Solifluction lobes and other peri-glacial features such as cirques are not glacial features per se. However, Bob did mention a probable moraine elsewhere in the New England, specifically, near Ebor in the vicinity of Duttons Trout Hatchery. A moraine IS a glacial feature. Because of these interesting features and because that part of the country is wonderfully beautiful I have wanted to do a road trip into the area but as yet have not been able to. The best I’ve been able to do is look at Google Maps but at least even consulting Google you can find some little gems.

A Google Earth image of the area to the North of Wollomombi
While looking at Google Maps I recognised more evidence of peri-glacial features in the Wollomombi area, which is about 20km to the south east of where the above-mentioned features were identified near Guyra. Here too was evidence of solifluction (movement of soil due to the partial thawing of summer permafrost). I’ve not been able to identify with certainty any other evidence of solifluction or related features even in the higher (and therefore colder) parts such as Ebor. Maybe, it was the case that during the last glacial maximum (about ten to twelve thousand years ago) only isolated areas formed permafrost - seemingly small areas of south facing hills.

However, when noticing the places where periglacial features are present such as east of Guyra at Malpas Dam and those I just noticed north of Wollomombi, I thought that they seemed only to be present on hills that looked like they had soils derived from basalt rock. Indeed, upon inspection of the geological maps it became apparent that the only places where I can see these peri-glacial features are mapped as being on Cenozoic aged basalts. The map shows the south facing hills that are derived from other rock types such as granites and meta-sediments do not show the same evidence of being affected by permafrost or related processes. This is interesting because there are two possible reasons for this:
  1. There was only isolated areas that were cold enough to maintain permafrost during the last glacial maximum; or
  2. The soils derived from granites and meta-sediments did not preserve evidence of permafrost
Given that the solifluction lobes evident at both Wollomombi and Guyra are about 20km from each other I would suggest that it is unlikely that the effects would only occur in these two areas and not in the area in between, so option 2 is the most likely. This may have the following implications:
  • Zones of permafrost (peri-glacial environments) and maybe small glacial environments probably existed in frequent patches on south facing slopes all the way between Guyra and Wollomombi and maybe even further to Ebor an area 60km long;
  • The soils in this area are derived from three major types consisting of Carboniferous aged Meta-sediments of the Girrakool Beds and Sandon Beds, Permian and Triassic aged New England Batholith ‘granites’ of the Abroi Granodiorite, Rockvale Monzogranite and Round Mountain Leucomonzogranite and finally Cenozoic aged ‘basalts’ including the Doughboy Volcanics and others which are unnamed;
  • Only the soils derived from the basalts have properties available to behave in a manner which produces and or preserve the evidence of permafrost in features such as cirques and solifluction lobes.
A Google Earth image of a spot next to Malpas Dam near Guyra.
Here the solifluction lobes are comparatively big
So, what does this mean? Well, it means that it was very cold over a large area in the New England. So much, that during the last glacial maximum, water was permanently frozen in the soil in south facing topographic areas over a widespread region extending at least from Guyra to Ebor. But, evidence for this was only preserved in the soils derived from basalts (I need to consult a pedologist (soil scientist) to figure out exactly why this might be the case).

So, if you are shivering and experiencing snow flurries in the area during winter, know that you would have been shivering harder had you been there about 20 000 years ago. It makes me wonder if the indigenous people of the region experienced that cold or whether the land was too cold and marginal for them to live there at that time.


  1. Can you be so sure these features in basaltic soils are peri-glacial? Couldn't gravity,occasional saturation of typically deep heavy basaltic soils and land clearing on steep slopes explain this lobing?

    1. I can be as sure as I can be by looking at Google! The short answer is no, I can't be sure but I think it the most likely explanation to fit the evidence.

      I feel that they are more likely to be freeze-thaw features rather than just saturation. The features are obviously only present on the south sides of valleys and hill sides. The opposite facing northerly aspect slopes even if comprised of basalt soils do not show the same lobing. Additionally, the lobes do not seem to be present in drainage lines but some features that look like cirques that instead may be what you describe are present in some drainage lines near Guyra. The lobes are present at locations you'd not expect to be very wet while there are very few lobes where you'd expect more waterlogging. If these were just due to normal water movement there would also be features showing active saturation around theses sites which does not appear to be the case (from the aerial photographs).

      Having a look in the field is by far the best way to answer this question... and I've not had a look in the field!

      Thanks for your thoughts, your idea could be the correct one after all... if only I could get the chance to see the area in person!

    2. How does one determine a cause for such features? Do you excavate,and are the actions of permafrosting distinguishable in cross-section from other kinds of processes that might periodically destabilise soils on steep slopes?

      What can one extrapolate from the current climate at Guyra? The coldest months average around 10C daily maxes,and around freezing to just above for minima. At the last glacial max we take off how many degrees Celsius? I suppose it's very likely soils in suitable shaded sites were frozen for at least 4 to 6 months of the year back during the LGM.

      I suppose 'permafrost' makes me think year-round frozen soil and that's not essential to produce solifluction features, is there a term for 'frozen for part of the year'?

      I had a look at Malpas on GE and noticed 'slumping' features on west facing slopes above the dam. My untrained eye is probably missing something,what do you see there?

      Thanks in advance for your reply, I really appreciate your work here which has given me extra eyes in my travels through the district!

    3. G'day Nick.

      Thanks for a thought provoking series of questions. They are giving my noggin' a work out.. and very nearly exhausting my knowledge of this area!

      Solifluction results in the 'defrosted' lobe over-riding the underlying and frozen materials. Whereas, lobes from slumping tend to take the underlying materials with it... though as you can imagine it is not always quite as simple as that.

      I've never dug a solifluction feature before but I'd imagine a trench through a lobe giving a complete profile will show evidence of the lobe over-riding the soil under - it probably includes buried organic layers. For a palaeo-solifluction lobe, I'm assuming this is a diagnostic feature.

      As for the are near Malpas, you can see some nice big lobes on the southern faces of the hills and valleys from the dam westward towards Guyra. There are also some depressions resembling cirques in that area too. The hill just to the north and west of the dam is a good example of what we are discussing. If you have a look at the older post you can see a photograph of it looking across the dam from the south.

      As for actual expected temperatures I'm not certain. I know there has been a lot of papers and thesis which discuss palaeo-climate. It has been suggested by many Authors from ANU and other unis that the area during the LGM was colder (though I cannot recall by how much) but also drier.

      Does this answer your question somewhat? I like having to think even more deeply about what I write and you've certainly provided a lot of extra deep thinking!

      Keep firing these excellent questions!

  2. This map may be helpful, select "Reliant on subsurface".

    Browsing it in the past I've noticed some scared areas around there, draining out of the ranges, that look ancient also correspond with groundwater dependant ecosystems. May help provide some clues and highlight areas to investigate.

  3. It may be significant that these apparent solifluction lobes only occur in association with basalt. The basalts on New England are generally composed of multiple lava flows with interlayered palaeosols. The fractured basalts hold and conduct water readily, while the palaeosols act as aquicludes. In these conditions, steep slopes are prone to slip circle failure, with the weight of basalt sliding on the weak, saturated palaeosol. There are a number of these land slips on the northwestern facing slope on the eastern side of the New England Highway as it nears the top of the Devil's Pinch (difficult to see on Google Earth as it is tree covered). One of these slips has been intermittently active over the 40 years I have been in the area, to the extent that it was necessary to use reinforced concrete pavements on the new highway in this area. So I am not convinced it is necessary to invoke periglacial conditions as an explanation. A search of the internet with "slip circle failure basalt palaesol" suggests it is a fairly common phenomenon in basalt areas with sufficient rainfall and dissected topography.

    1. Hi Anonymous,

      You are right in pointing out another mechanism that would make a similar mass movement pattern. It is likely that my speculation about the Wollomombi lobes in this post is indeed related to ground water discharge. Though there is still a remote possibility this might still be a peri-glacial feature.

      However, The southern facing hills in the Malpas area are a bit more interesting. I'm not yet ready to be convinced that the lobate structures on those hill sides are not periglacial. One reason why I think this is that there are many springs on the northern side of basalt hills in the area and these do not show the same shaped features as those at Malpas dam. I suspect that the northern sides were not cold enough to form these features, whereas the southern side was.

      Of course, this is all still some speculation on my part.

      Thanks for your thoughts on the matter. It is very good to look at other perspectives.