Fossil ice wedges

Ice wedges are formed in continuous or discontinuous (or sporadic) permafrost areas when the frozen ground contracta and when a wedge-shaped mass of ice then develops to fill the void.  Sometimes the wedge grows laterally and widens, pusing the flanking sediments aside and disrupting the pre-existing stratigraphy.  Later on, when a thaw sets in, the wedge of ice melts out, and sediments slip into the void and eventually fill it up.  These are called fossil ice wedges or "ice wedge casts" -- and they are a very good indicator -- in fluvio-glacial gravels in particular -- of very cold or tundra conditions which set in after the gravels are laid down. I have seen quite a few of these features in Pembrokeshire.

The top illustration is from the Mullock Bridge gravel pit near Dale, where the wedge formed in a Devensian kame terrace.  The lower example (from the old Mathry Road gravel pit near Letterston, Pembs) is not so well formed, and is best referred to as a "fossil frost fissure" -- it probably did not survive for more than a few years or decades.

The big question about these fossil permafrost features is this:  what age are they?   The gravels were probably laid dowm during ice wastage shortly after 20,000 years ago.  So the permafrost features might have formed shortly after that.........  On the other hand, there was a distinct cold snap referred to as the Younger Dryas, about 12,000 years ago.  That did not last for much more than a thousand years -- but it was certainly cold enough for permafrost to have formed in Pembrokeshire -- and indeed there is other evidence to support that contention.