As they died, so a deep layer built up, and slowly it became consolidated into rock. At a later date the sea-bed became dry land, as earth movements thrust it upward. With a little care these fascinating creatures can be re-discovered in the soft chalk. Only the soft material will yield whole shells. The chalk must be soft enough to mark the hand.
Cliffs are dangerous places, but look out for sites where the chalk subsoil shows through the turf.
Mole hills often contain nodules of soft chalk. The most effective method is to place the nodules in a wet cloth bag and thump them with a hammer. Then the white slurry can be washed through a 120 micron filter to take out the lumps of unbroken material then pour the milky liquid through a 25 micron filter to retain the shells and allow minute broken fragments to escape. The residue is washed in water until the liquid runs clear. A sample of material retained by the sieve can be taken with a pipette and a strew can be made on a microscope slide.