Erratic paradise

 

One of the great joys of our holidays in the Stockholm Archipelago is the exploration of small islands and skerries by kayak, discovering erratic conglomerations like the one above.  In places there are exposed, smoothed and striated rock outcrops visible along the shoreline, and in places the shoreline is covered with accumulations of erratics that have come from multiple locations.  There are very few morainic constructional features that one might refer to as terminal or recessional moraines.  The litter is everywhere -- made very visible and prominent because the whole landscape is "washed" as a result of ongoing isostatic uplift or recovery.  Many of the low skerries have literally emerged from the sea within the last few centuries, as evideenced by intermittent raised shorelines of cobbles and gravels.

This whole archipelago was deeply submerged beneath the ice of the Scandinavian Ice Sheet on several occasions during the Quaternary.  There was a great deal of areal scouring and very little erosion by identifiable ice streams -- although it is clear that basal ice did move more rapidly in places in response to topographic irregularities on the bed.  For example, the deep channels between the islands of Blidö and Yxlan must have enouraged streaming flow and must have led to localised variations in the direction of basal ice flow -- which was overall directly north to south.

In all of my coastal observations I have never been able to trace a true earratic train for more than a few tens of metres.  The abiding impression is that every boulder and cobble has a unique history -- sometimes moving, sometimes stuck, sometimes travelling directly southwards and sometimes in zigzag fashion dependent on topographic controls and oscillations in basal ice temperatires and flow characteristics.  And while, during any given glaciation, there must have been entrainment of fresh blocks on the glacier bed, many blocks such as those in the photo above must have been moved, embedded, and moved again over a series of glacial episodes.  

It seems to me that this process of entrainment and debris recycling goes on regardless of where the glacier snout is positioned.  This is an important point, since it means that in any given glaciation of the Bristol Channel old erratics can be stuck or dumped virtually anywhere, and new erratics can be entrained almost anywhere, right up to the ice edge as long as the ice continues to be active.  There are a number of different mechanisms.  For further details, see the standard texts............

In response to this constant process of entrainment, dumping, recycling and renewal, the majority of erratics on any ice sheet bed at any given time will be rather local, some will have travelled modest distances (tens of kilometres) and a few will be far-travelled (maybe hundreds of km).

This generalisation confirms what I have observed in the Quaternary sites around the Pembrokeshire coast.  It's rather futile to do a numerical / statistical analysis on this, because local circumstances vary so much, but in till (or on storm beaches such as Abermawr, where almost everything is derived from destroyed glacial deposits) MOST of the clasts are local, SOME of the clasts have come from more distant sources (such as Ramsey Island), and A FEW have travelled really long distances (such as Ailsa Craig).  

Newgale storm beach, where the great majority of stones have come from destroyed glacial deposits during the inexorable advance of the sea across the land surface.