One of my raised beds is like that.
The layers as you called them are called horizons this is the dividing line between the soil layers.
It has always intrigued me because they are so distinct and abrupt.
To me it shows something changed very fast in geologic time.
I used to work with a guy for a long time and one day I found out he used to be with the US geologic survey and had a degree in geology.
So I asked him about the strange round river rocks that are on top of the soil where I live and in other places deep sand.
This is hilly country not flat ground.
The rocks you would see in a river bed not up on hills.
He told me it was more than likely caused from massive flooding from glacial outflow.
Nothing you can find on line because everything is too simplified.
As for the raised bed I dug a big hole because I needed soil so I filled it back in with wood sticks and leaves and built a raised bed on top of it.
Worked out great but be prepared to add more soil as the stuff rots.
A thousand years from now someone will be baffled as to what it is so add some old bones and trinkets to make them think it was a burial site.
Worth