General   (General discussion, talk about anything.)

When I lived in Lincolnshire, I could dig about three feet in my garden to encounter shell bearing sand.

When considering geology though, you have to be prepared to consider very long time scales. In this case, the theory is that once upon a time the Lincolnshire Wolds were continuous with the Yorkshire Wolds. They hence dammed the water courses, to create the inland sea: the putative source of this sand. The inland sea disappeared when what we now know as the Humber breached this barrier.

Depth is not a reliable indicator. Just think of the coal seams: remnants of what was once tropical, but they outcrop in Orrell and in Bryn.