It's an old idea that the earth started smaller and grows layer-by-layer. A coat gets added, and vast amounts of time pass, and then another coat. Coats are usually added by worldwide floods, sometimes volcanic activity. The world gets wiped clean and life springs up again on the new coat. There are variations, of course. Earlier compromisers claimed the uppermost coat would represent Noah's flood.
Evolutionist ''geology'' to this day is a modified version of onion coat, and still easily recognizable as such. They imagine one way or another the same ''layers'' existed worldwide, being added at the same time, for example. They have all manner of flips and twists and ad hocs to explain away what's actually seen.
Evolutionism never had any problem with the flood(s) until spontaneous generation was debunked. Then they had to redo everything with no floods, insisting instead that sedimentary rock be the result of dry accumulation. ...And people believed they believed it!
That's probably more summary than you bargained for.