More on Lake Maw (Somerset)

In 2015 I posted these two spectacular images from Russell Glacier in West Greenland, showing partly-drained pro-glacial lakes.

https://brian-mountainman.blogspot.com/2015/11/pro-glacial-lakes-russell-glacier-w.html

I am now rather intrigued by the question:  could something similar have occurred in Somerset at the end of the Anglian Glaciation? (Or maybe at the beginning of it?  Remember that the biggest damming of Lake Teifi in West Wales is now thought to have occurred as the ice of the Irish Sea Glacier arrived -- not as it wasted away......)

There are fantastic details on these images, including faint shoreline traces related to earlier water levels, the shoreline or washing limit created by the highest water stillstand, and the surprising survival of pre-lake landforms and sediments after submergence for an unknown length of time.  I'm also intrigued by the apparent lack of thick lacustrine deposits on the old lake bed.  What we see is a litter of erratic boulders which might have been emplaced by the glacier when it was in an expanded state -- pre-submergence -- or possible emplaced as dropstones from the abundant floating ice debris such as we see in the lower photo.

Here is an interesting article:
https://www.researchgate.net/publication/233495943_Glacier_advance_ice-marginal_lakes_and_routing_of_meltwater_and_sediment_Russell_Glacier_Greenland

Most lakes of this type last for decades rather than centuries or millennia.  Details are given in this and other studies of ephemeral lakes relating to the short-lived oscillations of the ice edge, the routing of escaping meltwater across cols (where overflow channels may be created) and the sediments transported and deposited.  One particular focus of interest is the manner in which jokulhlaups (catastrophic drainage floods) occur -- but shoreline and lake bed features have received much less attention.

Here is another image from West Greenland, showing pro-glacial lakes (with cloudy or sediment-rich water)  impounded as a result of the details of local topography.  Sometimes lakes are connected together in strings, and sometimes there are complex connections across submerged cols.  The retreating ice sheet edge is on the right.  Note the uplands with small glaciers and snowfields (with clearwater lakes) in the bottom left quadrant of the image.  We can pick up the main drainage routes leading from the pro-glacial lakes towards the coast  -- along these routes sediments are deposited and terraces may be formed.

Back to Lake Maw.  On the map by Prof Nick Stephens he suggests that the Saalian / Anglian lake (if it really did exist) was impounded by an ice edge running along the middle of the Severn Estuary and located maybe somewhere near Flat Holm.  He suggests that most of the water in the lake came from melting on the ice front and from the impounded or trapped Severn River, submerging an extensive area across the Somerset Levels including the sites of Bridgwater and Burtle (where the famous Burtle Beds are located). He suggests that Cheddar and Weston Super Mare were not submerged, but if the water level was above 82m -- which it must have been, for water to flow over and through the Chard Gap -- then the lake must have been much more extensive on its northern flank. 

Clearly, the nature of the sediments beneath the peat beds in the Somerset Levels are critical in the arguments about the existence of Lake Maw -- I'll take a look at that particular matter in another post. 

A fine photo by Andy Russell of two of the Icelandic pro-glacial lakes held up against a hillside by very dirty ice -- and in the process of breaching the ice bridge that separates them.  Soon there will be just one lake here -- unless a catastrophic breakout occurs, in which case there will be a flood and the lake(s) may drain completely.