In the last Ice Age there was an ice cap from which ice sheets and glaciers travelled south, eroding huge amounts of material on their way. This material was redeposited as till or boulder clay across much of the lowlands of Northern England. When several valley glaciers came onto a lowland plain, they spread out and joined into a single ice mass called a piedmont glacier. When the ice melted about 12,000 years ago, temporary lakes were formed in which deposits of fine silt and clay accumulated. Laminated clays of this sort have been used for brick or tile making, in the area which is now the nearby Tilesheds Nature Reserve.
Further information can be found at an information board at the above coordinates. In order to log this cache please take a photo of it or the island with your gps and email me the aproximate thickness of the ice sheets that traveled to make this feature.