The ‘overburden’ in our area typically is 2 to 5 feet. Don’t know what it is elsewhere.

The earth’s crust is being constantly recycled, as tectonic plates subduct surface rocks into the mantle, and vulcanism brings up magma from the depths to deposit as lava on the surface. But these processes are extremely slow on a human scale.

The earth’s surface is constantly being worn away by erosion, and the removed material is being deposited somewhere else, burying previously exposed strata. And this refers directly to your question.

The effect you’re referring to, affecting archaeological artifacts and fossils, tends to bury those items ever deeper, while if they are exposed, it is only briefly, before they are scattered about or weathered away. This is why most of the stuff we dig up is buried by deposition, if it had been revealed by erosion it would soon have vanished altogether, destroyed forever.

The reason these objects are so rare is that it generally takes a combination of these processes to bring them to our attention. For example, a river valley frequented by dinosaurs collects the remains of many dead animals, which are deposited by floods and currents in one place, and covered up by mud and sand. Millions of years later, those buried strata, now compressed into rock, are eroded by water or wind and exposed just in time for a scientist to walk by and see bones poking out of the dirt. Valleys, banks, road cuts, cliff faces and other recently exposed deep sedimentary rocks are great places to look for fossils, but you have to be quick, every year more stuff is eroded away and any fossils you miss this year will be gone by the next.

When I was kid, our favorite place to look for fossils was at the phosphate pits, open pit mines where old ocean bottom was excavated for fertilizer-rich rocks. We found shark, skate and crocodile teeth, fragments of whale ribs, and other marine fossils. When I worked on the golf course construction crew, the fairways, tees and greens, recently bulldozed free of all vegetaition, turned up all sorts of native artifacts; flint flakes, chips and cores, projectile points, scrapers, pottery fragments and clay fishing sinkers.

In either place, the most productive time to go looking was right after a heavy rain. It always seemed to reveal new stuff

In cities, new construction is always on top of the rubble of the old. In ancient towns, like you get in the middle east, the deeper you dig the further back in time you go. Every new building is built up from its surroundings for drainage, so the city tends to rise, like a layer cake. When you do any major excavation in an urban area, for a subway or building foundation, you’re going to turn up old stuff. But most of the material you are removing from your hole is not geological, it is old masonry, powdered and ground up into soil by the centuries.

Of course, sometimes major construction moves the material out of time sequence, giving future archaeologists fits. We know the Greeks used Egyptian rubble in their construction, and the Romans used the remains of Egyptian and Greek buildings for their own. The Arabs did the same later. A modern Cairo building may have material from all these cultures incorporated into its foundations.