It's not just mud.

This exposed area of mud uncovered by the tide is in fact the remnants of  the prehistoric shoreline of this area. Animal and human footprints have been found over recent years in sediment exposed at various times.
Here is a link to more information if the subject is of interest.
http://www.formby-footprints.co.uk/index.html
I have added a couple of extras of a more scenic nature.