A to Z of North Wales Geology
T is for..............TILL (ERRATIC)
can include rock fragments of considerable size. Pieces this big (left) are less common, its
longest dimension is about 4 metres. The scale can be judged from the author's cycling bag at
This superb erratic block sits at the end of Deganwy Beach by the Cerrig Duon breakwater. Grid Reference SH773806. It is so large that it has behaved rather like an outcrop, and bears the scars of other boulders being dragged over it.
If you visit the site at low tide, you will see some of the rocks which give the locality its traditional name. Cerrig Duon translates from Welsh as the Black Rocks. Sadly, most of the dark, weed encrusted boulders were buried during the construction of the groyne in the early 1990's. The remnants are scattered across the beach platform beyond the groyne.
It is probable that the jutting outcrop of till is the eroded remnant of a drumlin, which is thought to be the site of the ancient fortification of Castell Tremlyd (N.Bannerman, Bronze Age Coast Project).
DRUMLINS are elongated hills of till and other glacial debris which apparently form beneath flowing ice. I won't say any more, since the book I am using for reference runs to 30 pages on the difficult subject of drumlin formation. If you wish to look at a really good one, go to Dinas Dinlle near Caernarfon. It, too, is in an advanced state of erosion by the sea, and is surmounted by an Iron Age fortification.
©Jonathan Wilkins, 07.00