Trace fossils probably burrowing molluscs impressions left in Carboniferous sandstone, Heald Town Quarries, Weir, Bacup. BGS: Dyneley Knoll Flags - Sandstone. Sedimentary Bedrock formed approximately 312 to 313 million years ago in the Carboniferous Period. Local environment previously dominated by swamps, estuaries and deltas. Setting: swamps, estuaries and deltas. These rocks were formed in marginal coastal plains with lakes and swamps periodically inundated by the sea; or estuaries and deltas, and shallow seas.