Red Crag
As the Norwich Crag is a marine formation, the majority of fossils found in it are of marine origin; any terrestrial species were originally blown or washed into it from land, or derived from earlier deposits, particularly the Red Crag. Vertebrate fossils tend to be concentrated in the basement bed or in gravel lags. Species of warm and cold substages are represented, sometimes mixed together; this may pose problems for palaeoecological interpretation.