Hi Michael,
 
in the south of Paris is Fontainbleau - in this region are fossil depots.
Recently I met a palaeo-digger - he told me, that there after ca. 40 cm
humus  is the pure Eocene beach. He has found big Campanile gigantea, but
also many better conserved interisting shells.
The digging is time and power consuming, after the harvest in this region
you can find also shells on the acre or after heavy raining.
 
Good luck,
Ingo