One of the most spectacular karst sites in Britain is the Norber erratic field in the Yorkshire Dales. A glacial erratic is a rock transported by ice, left behind on a different rock type. Retreat of ice at the end of the last ice age left behind boulders of Silurian (444-419 million years ago) quartz-rich greywacke on top of Carboniferous (359-299 million years ago) limestone. The field of boulders is the result of plucking from a greywacke outcrop from a short distance away. Only one boulder has evidence of travelling at the bed of the glacier. The greywacke is very resistant to erosion, but the limestone gradually erodes because rain and soil is slightly acidic because of dissolved carbon dioxide and organic acids. Through time, the limestone surface erodes away but the greywacke block protects the limestone underneath, which slowly forms a pedestal.
Weathered limestone pavement consisting of clints (blocks) and grikes (fissures), Thwaite Scars
Norber is one of the oldest sites in the UK where there have been attempts to date the end of the last ice age. At the end of the 19th century there was considerable debate as to whether there had been glaciers in Britain. It was clear from the evidence of erratics that there had either been much more extensive ice in the past, or that a flood covered Britain and sea ice had dropped the blocks. The popular latter theory (now thoroughly discredited) led to the term “drift” being used to describe glacial deposits, which unfortunately is still used today. A Fellow of the Geological Society, D. Mackintosh, read a paper to the Society in 1883 in which he reasoned that since the Norber erratic pedestals were about 6 inches high and that the rate of lowering was 1 inch in a thousand years, the ice age ended here 6,000 years ago. The estimate was criticised on the two grounds. First the rate was highly variable locally and poorly constrained. Second, it didn’t seem believable from known history that the ice age was so recent. It was pointed out that if precession of the equinoxes was the reason for the ice age, the maximum cold was likely to be 10,500 year ago (which was half right).
It is important to know the rate that limestone weathers to improve models of landform evolution and soil formation. It is difficult to measure the rate of limestone weathering physically because you need a reference (such as a quartz vein) and a known time the reference was made or exposed. References such as metal objects can be used to measure modern rates but it takes decades to make good measurements. Cosmogenic nuclides provide a modern method to determine weathering rates. This can be done in two ways. The first is to find a site that has been weathering for a long period and steady state has been achieved (between the nuclides being created and those being removed by weathering). The second is to find a surface created at a known time and measure the apparent age of the eroding surface, then turn the age difference into an amount of weathering. The advantage of the second method is that it works on newly created surfaces but it is tricky because it relies upon finding something to date the creation of that surface. At the Norber erratics, this condition is satisfied because the blocks have not eroded and can therefore be directly dated using exposure dating.
As part of a collaboration with Peter Vincent and Peter Wilson1, we made a modern attempt to date the Norber erratics and determine the rate of lowering. I analysed samples of limestone near the Norber erratics for cosmogenic 36Cl. This isotope is produced from interactions between cosmic rays and calcium in the rock. We calculated that the erratics were deposited 17,900 years ago. The limestone surfaces had a younger apparent age. The age difference is the equivalent of 33 ± 10 cm of limestone erosion. Combining the two sets of data together reveals that the rate of erosion is the equivalent of 18 mm per thousand years (or 18 m over a million years). Finally we had enough data to put to bed a century old mystery at the Norber Erratics.
1. Wilson, P., Barrows, T. T., Lord, T. C. & Vincent, P. J. (2012). Surface lowering of limestone pavement as determined by cosmogenic (36Cl) analysis. Earth Surface Processes and Landforms, 37 (14), 1518-1526.