The Noel Hill residual raised beach

 

This photo was taken on 29th January 1966, not far from the summit of Noel Hill on the Barton Peninsula, King George Island, South Shetlands.  This is not the highest beach found on that day, at 275m asl, but it illustrates the complexity of working in this sort of environment.  

I'm standing on the edge of a raised beach deposit, made up for the most part of small rounded beach cobbles.  This is where one has to record the altitude, since it is a clear junction between a blocky till bank to the left and an undulating beach surface to the right.  So is this a washing limit?  Well, yes and no. If we look at the till bank we can see that it has been cleaned up by the waves, and that the finer matrix material has been washed out and carried away, for at least a couple of metres above the ground surface where I am standing.  So the real washing limit -- and hence the maximum elevation of relative sea level (rsl) -- is higher, but we don't know exactly where it was. 

So my feet mark the position of a stillstand, where the relative positions of land and sea were stable for a number of decades or centuries.  This is both an erosional and a depositional feature --  there has been washing of the till face or edge, abd some undercutting of the till deposit, and there must be more boulders associated with the till buried beneath and within the raised beach terrace.

The reason for the stillstand is difficult to discern. There may have been an equivalence in rates of isostatic uplift and eustatic sea level rise, or an equivalence of rates of isostatic depression and sea-level fall! Tectonic factors may also have played a part, as David Sugden and I speculated in our big article in 1971:
John, B.S. and Sugden, D.E. 1971. Raised marine features and phases of glaciation in the South Shetland Islands. British Antarctic Survey Bulletin, 24: 45-111.

David and I were certain that these high "residual beaches" on the Barton Peninsula and elsewhere were covered by a short-lived glacier advance.  Some beaches will have been destroyed, while others survived.  So the ice advance was not a very powerful one.  But it was strong enough to erode and incorporate many raised beach deposits into a patchy till cover.

Cosmogenic dating of large cobbles incorporated in one of the lower Noel Hill residual beaches by Emma Watcham gave dates of around 7,000 yrs BP.  She could not match these dates against other evidence relating to the glacial chronology for King George Island, and so concluded that the residual raised beaches are not beaches at all, but remnants of destroyed kame terraces.  I disagree.  For the kame terrace interpretation, we need much more evidence.  From the hundreds of sites we examined in the South Shetland Islands, we found no evidence for substantial meltwater deposition anywhere.  The beaches range in altitude between 275m and c 150m asl -- some are to the east of the hill summit, and others to the south-west.  There are no signs of terraces that might have been formed between a hillside and a downwasting ice margin, and no signs of catastrophic ice wastage with huge volumes of meltwater refashioning the depositional landscape.  All the signs -- as far as we could see -- pointed to a very modest amount of meltwater activity and to very subtle changes in the relationships of ice and land surfaces at a time of very high -- but falling --relative sea-level.

See also:

WATCHAM, EMMA,PEARL (2010) Late Quaternary relative sea level change in the South Shetland Islands, Antarctica, Durham theses, Durham University. 

Available at Durham E-Theses Online: 

http://etheses.dur.ac.uk/485/