A memory of the 3D shapes which were once beneath is absorbed by the paper.
I thought I would add a link to Locard’s principle as well-mentioned by Heather in comments below – fascinating.
https://en.m.wikipedia.org/wiki/Locard%27s_exchange_principle
Sally Wakelin - Studies in Creativity 
Love this Sally, both visually and in the sense of how it directly and yet poetically links in the forensic sense to Locards principle that ‘Every contact leaves a trace’. Heather
Thanks Heather, that is beautifully put. I am of a mind to do more but I need paper with longer fibres to get a little more depth. I think lithography paper might be worth a try.