On ancient shorelines

When I was a young researcher, I spent a lot of time hunting for shorelines -- in Iceland, Greenland, Antarctica and Norway -- and in the UK as well.  If one can find them, they can tell one a lot about events related to glaciation in general, and glacier retreat in particular.

Raised shorelines can either be related to  sea-level (if there is access to the open coast) or to pro-glacial lakes, which tend to form in "closed" situations where water levels can oscillate rapidly and where catastophic drainage can sometimes occur.  Quite literally, one year an ice-dammed lake  maybe 200m deep may be there, and the next year it's gone.  Some lakes can fill and empty over and again, causing some confusion in the interpretation of shorelines or "washing limits."  But the general rule is that the highest shorelines are the oldest, and the lowest ones are the youngest.  Another rule is that where there is isostatic readjustment going on, the highest shorelines (if you can trace them along many miles of a shore) will have the highest tilts, and the lowest ones will be increasingly closer to horizontal. Where you find sets of shorelines, they can intersect.  In general, if you find a shoreline you assume that it represents some sort of stillstand which coincides with a stabilisation of ice volumes over maybe decades or centuries.  But the speed of development of a shoreline will depend on the amount of wave action on a shore, the annual length of the ice-free season (when the surface of the water is free of ice), the fetch, and the nature of the shore itself.  A sediment shore will leave traces of a stillstand far better than s steeply-sloping clean bedrock shore.  You have to think in all dimensions -- and interpreting shorelines involves a highly complex sort of multivariate analysis.

The pictures above show the 3 shorelines of the Parallel Roads of Glen Roy, in Scotland, a set of shorelines alongside a pro-glacial lake in South Georgia, and a shoreline (yes, that's me in 1966!) in the South Shetland Islands, Antarctica.

This info is all relevant when it comes to trying to work out where the Devensian ice limit was in SW Britain, since there is much talk of lakes particularly on the southern coasts of Cardigan Bay. (See the earlier posts relating to the South Wales End Moraine, Glacial Lake Teifi and "Lake Brynberian"........)

In spite of years of hunting, I have never found anything that I would call with any certainty a glacial lake shoreline, either in the Teifi Valley or the Nevern Valley.  I'm pretty sure that this means the glacial lake inundations were VERY short-lived, with no water level static enough, over a long enough period of time, for the formation of shorelines.  As I have suggested earlier, the answer may lie in the varves.  But I'm still putting my money on the ice remaining at its maximum position for no longer than a few decades.

Another complication, with Lake Teifi and maybe the other North Pembrokeshire lakes, was that maybe the oldest shorelines were lower than the youngest ones -- with the suite of shorelines representing a gradually rising water-level as the ice advanced across valley mouths and meltwater accumulated to higher and higher levels as the ice dam became stronger.