"Deep time" at Red Wharf Bay

In his introduction to the re-print of Clarence Ellis's book The Pebbles on the Beach, Robert Macfarlane says that geology gives him a sense of what he calls "deep time". Ellis was born on Anglesey and it was on Anglesey that I got the sense of "deep time". We found these alternating layers of limestone and sandstone. The former was laid down in tropical seas and then the land rose, leaving rivers to deposit the sand and swamps the black mud shale deposits. Human history goes back in the thousands of years but I get the impression that each layer must represent a time longer than that. I'd be interested in finding out how long a time each layer represents and how you would work it out.