If you dig a hole, you puncture layers of sediment. Do it carefully, and you'll be able to see those layers of sediment on the walls of the hole. But the dirt you extract is mixed, and homogenized in the process. Fill in the hole, and the fill won't have nicely stratified layers. If somebody carefully digs in the same place 1000 years later, a cross section will reveal a homogeneous plug amid layers of strata. They'll be able to date when you dug the hole by looking at the layers above that homogeneous plug

Fun thought: archaeologists 10k years from now unearthing today's digs, finding the traces of our careful destruction of ancient evidence...