Sinking medieval churches

I am reading Bill Brysons's book about the history of everyday living. It's packed with interesting snippets. He's basically done lots of research and pulled out all the interesting bits for the reader. My kind of book. It's turned me into a walking verbal fact box much to the delight of chap. I pop up regularly with an urgent need to tell him something new I have learned. Of course I'll expect him to read the book afterwards too.

The part I have enjoyed most so far, however, I found on the first few pages. Bill B lives down the road from here in a Victorian rectory. One of our many 'where shall we go for a walk - let's look on the ordnance survey' walks in the Norfolk countryside runs between his house and the church he mentions in the book. And here is what he records from a conversation with a historian while wandering around that church (paraphrased):

Have you ever wondered why Norfolk's medieval churches (all ten million of them) always look like they are sinking into the ground? Careful: it is a trick question. Of course I have always just thought, 'well a stone church is quite heavy, medieval foundations were probably not overly sound and they've had centuries for gravity to shift them about a bit'. But no, the revelation is that it's not the church sinking, it's that the graveyard has risen. The hundred or so gravestones that are typically found in such a graveyard belie the volume of dead matter buried there. The book goes on to explain that with a parish of around 250 people (4 generations per century plus numerous baby/childhood deaths) you would be looking at around 20 000 burials. I could raise a two foot platform the area of a graveyard with that many skeletons. I wouldn't want to of course.

Now that is interesting and it has definitely changed the way I look at all those sinking churches. But I also would like to learn about grave diggers. How did they go about their business with all those bones everywhere? Making a 'new' grave must have been an exercise in clearing out lots of old bones. If so where did they put them? I'm sure the bereaved were not subjected to a little pile of bones at the graveside to be replaced once their loved one had been tucked away? And, presumably some graves were more shallow than others or is all this dead matter always more than six feet down? As ever, more questions than answers. Another book please Bill.