Those stone roofs around lake district..

I noticed when up the lakes area a bit back those stone roofs.Starting at the eaves with large flat stone slates,then getting smaller and smaller up to the ridge.Is it done this way just because there are many more smaller stone slates around (cheaper)How are they held on,quite a few looked a right old mess with cement holding them together.

http://www.tewkesbury.gov.uk/media/pdf/g/1/Slate_Roofing_.pdf the tiles have holes bored through and oak pegs inseted to hook over the batten, no reason i can think of( unless its a weight thing) for the deminishing courses

Random and diminishing,random widths and diminish as they go up the roof.I have stripped a roof of and the slates were fixed with a mixture of wooden pegs and bones but these were the big sandstone slates(lazenby slate).