Lenses to look back in time

Not an ispiring image but to my mind, an interesting one.
Us archaeologists (well I try....) call this a lens - a deposit sandwiched between two other layers. This one is in a vertical deposit on the shore at Baleshare that has been exposed by the recent storms.
Hadn't really thought about it until someone mentioned it - this particular  lens (or at least the section on the left) is essentially one meal of cockles that someone then tipped onto the rubbish pile. Not very inspiring until one realises that meal was eaten almost two thousand years back in the past. 
I imagine the word lens comes from the typical lozenze shape of these deposits but it does seem appropriate to use it in the optical sense too.