djunna
My understanding is that this type of memorial is known as a Grave Slab or Grave Cover. There are several examples of this in our area - the North East of England (Derwent Valley) - and several different kinds. They are all typified by their size and mass - usually approximately 6 feet long by about 3-4 feet wide and 6 inches thick. They probably weigh in the region of at least a quarter of a ton.
The example attached is a memorial to one of our local parish priests and is relatively recent, 1915. I think it completely fulfills the purpose of any good memorial in that it is decorative as well as being very informative. Another one locally was about the same size, in a different graveyard, and completely covered in thick moss, apart from one small area which had a couple of digits incised on the surface. These looked like the last two letters of a year date so I set to and cleared it all off. This revealed a full, and very clear, inscription, with no other decoration. For those purists who decry the removal of obscuring growths I can report that within 6 months the moss had completely re-established itself.
I think it's possible that these covers may be used when the family decides that no further burials will be made in these plots and the slab/cover may be seen as a final seal.
PatB.