Glacial erratic at Norber Brown that sits nicely on a limestone plinth, dues to the erratic’s having protected the limestone underneath from erosion. (credit: Wikipedia)
Few expect Earth scientists to get all sentimental, but they do. My soft spot is for one of the most rewarding and least strenuous geological sites in Britain, Norber Brow near Austwick on the southern edge of the Yorkshire Dales National Park. As well as the famous glacial erratics of Silurian greywackes perched on Lower Carboniferous limestone, 250 m to the SE by a well-trodden path is the inverse, the Variscan unconformity at the base of the Carboniferous on the very same Silurian formation. I was lucky to be taken there at age 15 by Roy Happs who taught A-level Geology, and it decided my future, there and then.
The erratics don’t just site on the limestone, but are on pedestals up to 30 cm above the surrounding limestone surface as if carefully balanced by Beowulf’s assailant Grendel. Somehow, since the time glacial flow had deposited the Silurian boulders the underlying limestone had been dissolved away; but how fast was that? That is the key to the pace at which limestone pavement, to most general visitors such a stunning and unexpected feature of the Dales, might have formed. And such a delight to hear of its terminology: clints, redolent of the former Viking people of the Dales, that stand proud between deep fissures known as grikes, a suitably ominous term of unknown derivation. Such superbly fractal landforms are, of course, but one part of karst (from the eponymous region of limestone country in Slovenia).
A classic limestone pavement in the Yorkshire Dales National Park (credit: Wikipedia)
It is really satisfying to discover that a lot of cutting-edge science has recently been aimed at Norber from a substantial review in Earth Pages’ sister journal Geology Today (Wilson, P. et al. 2013. Dating in the Craven Dales. Geology Today, v. 29 (January-February Issue), p. 16-22). The length of time that the Norber erratics have been exposed to cosmic-ray bombardment has been determined from 10Be, 26Al and 36Cl analyses with a precision of ±1000 years to 17.9 ka, shortly after the last glacial maximum (LGM) when warming and glacial melting had just begun in this part of Yorkshire. That might seem to indicate an average of 330 mm of limestone had been dissolved over that period to form the pedestals, i.e. a dissolution rate averaging about 20 micrometres per year, which is extremely rapid, geologically speaking. In 1962 when I was show the site we were told that elsewhere the limestone pavement had formed since the first field systems (Iron Age) were laid out as now useless drystone walls crossed it. Roy Happs somewhat darkly suggested that they had formed since the start of the Industrial Revolution because of acid rain.
He was pretty much wrong on that score, but cosmogenic dating of the clints shows significant discrepancies between the age of deposition of the erratics and and the exposure age of the clints. This suggests both chemical dissolution and also periods of frost shattering and gravel removal, perhaps by soil creep. Dating of other materials enlivens the history of local landform development. Another karstic feature is the presence of sinkholes or dolines that are often filled with yellowish silts that show clear textural evidence of being windblown sediments or loess. These aeolian sediments have long been regarded as post-LGM too, but optically stimulated luminescence dating of their quartz grains gives an age split between pre- (27.5 ± 2.6 ka) and post-LGM (16.5 ± 1.7 ka). Some loess elsewhere in Craven district comes out to be as young as 8.2 ka, to tally with evidence from Greenlandic ice cores for a sudden deterioration in North Atlantic seaboard climate during this early time in the Holocene.
Then there are the local caves, renowned in Victorian times for their cave bears and other mammal fossils. One bear skull from Victoria Cave in the Craven area gave a 14C age of 14.6 ± 0.4 ka which statistically coincides with that from a cut-marked horse vertebra. More than likely the bears were turfed out when humans reached Craven, but did they return when humans fled in the face of the Younger Dryas return to frigid-desert conditions? Probably not, as the YD would almost have sterilized what are now the Yorkshire Dales. Even earlier ages of 114 ka from U-Th dating of calcite flowstone that embeds hippo, elephant, rhino and hyena bones in Victoria Cave date to the previous Eemian interglacial. Indeed this speleothem has yielded ages as far back as the limit of the U-Th method (%00 ka). On a solo expedition in 1964 I had the chance to sleep-over in Victoria Cave, but pressed on with goose bumps to the nearby Youth Hostel.
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This is not my subject, and I therefore speaking as a fool rushing in where angels fear to tread, but I’ve long had an interest in karstic landforms, indirectly through the work of karst geomorphology specialists in the University of London colleges in the 1960s. One argument at that time, was that well-developed limestone pavements in Britain are confined to areas in which in the associated strata (of ‘Carboniferous Limestone’ sensu lato) were more or less horizontal , and (2) areas that were under ice during one or more of the Pleistocene glacial maxima. I know that other factors are relevant too, but the geographical pattern strikingly compares well with glaciation limits (http://www.limestone-pavements.org.uk/distribution.shtml), especially bearing in mind the absence of pavements on limestones of Devonian, Mesozoic as well as Carboniferous age in southern England (e.g. Mendips, which otherwise has spectacular karstic features like Cheddar Gorge). The older ideas, argued long before the modern benefit of cosmic particle dating, suggested that ice action scouring across the relatively more resistant limestones of the eventual pavement-strata, was a major factor. The dates mentioned in this article, very close to the LGM at 18 Ka, would at least be consistent with these older theories. Of course, additional theories (as in the present article) are needed to explain the various patterns of clints and grykes.
I am sure that you are correct as regards the need for ‘ground preparation’ by glacial scouring to encourage limestone pavement development in Britain. That would scour off any cover by pre-glacial soil or tills deposited by earlier glacial episodes. There are, as you probably know’ plenty of places in Northern England where pavements do not occur thanks to till having been plastered over large areas of flat ground above ‘subcrops’ of Carboniferous and other limestones. Yet such areas generally have their full complement of sinkholes, some inherited from pre LGM karstification (there is a cracking example in the dry valley below Malham Tarn, which is partially clogged by till). As regards the clint and gryke patterns, they seem controlled by joint patterns and also the nature of the limestone strata themselves. There is a clear difference between pavements developed on thick, massive limestones and those with rapid alternations of limestone with shaley partings. Beyond the extent of the last full glaciation surface processes were dominated by solifluction probably activated each summer as near surface-seasonal melting occurred above impermeable frozen substrate. The periglacial ‘head’ deposits of southern Britain probably sealed shallow carbonate strata from infiltration and solution. Yet, I am sure there are some pretty good limestone pavements in the type area for karst in the Balkans, which never underwent glaciation.
Thanks for your reply – very interesting. At the risk of prolonging this —-way back, I was an informal ‘field assistant’ to a friend working on the karst features of Cowside Beck near Malham, attempting to separate possible glacial control from structural control (substantial limestones like the Great Scar, almost horizontally disposed). BTW Hutton Roof is my personal favourite though. Your comments also made me wonder about some of the Alps we visit, with huge thicknesses of (e.g.) Triassic limestone, some under ice all the time, some exposed only in the summer and some formerly under ice but no longer covered by ice at all – a natural laboratory perhaps. I’ve not seen pavements there (so far) though, just masses of frost-shattered topography and/or roches moutonnées with frost-shattered scree everywhere! But Wikipedia shows a pavement in an Alpine setting near Grenoble. In tropical places I’ve been too, it seems we get murderously jagged ‘champignon’ (etc) instead of real pavements, with biofilms a key part of the weathering process I gather, so, yes, climate in various ways has a big role in all this, and on different scales. Re clints and grykes, I believe structural geologists have shown (e.g. at Malham) that the grykes are effectively karstically widened joints and small faults whose directions can be ascribed at least in part to local much older tectonic events, there being an almost standard relationship between directions and stress fields for a relatively uniform thick (competent) rock under deformation.