Cesium age dating
The sediments in Sky Pond in the Rocky Mountains show that around 1950 nitrogen began to flood the lake, an event unprecedented in at least 14,000 years and this isotopic record too could have a pretty precise start date: July 2, 1909 when Fritz Haber first demonstrated how to make ammonia from the air in Germany.
In Greenland lead isotopes in ice cores reveal a record of lead pollution from Roman smelting in Spain some 2,000 years ago.
And climate changes, like the shift from the icy world of the Pleistocene to the summery clime of the Holocene epochs, have marked some past shifts in the geologic record.
Uniquely spherical magnetic minerals wafted over the world by coal burning can be found from peat bogs to lake sediments and may furnish a record of this carbon combustion for future geologists.
And their geologically abrupt disappearance makes them the perfect fossil to mark the end of the Ordovician, or so think geologists like Jan Zalasiewicz of the University of Leicester who has spent much of his professional life studying the beautiful shapes left behind by these long-gone animals.
Future geologists will also find a steady shift from tree pollen in sediment cores and other records to the pollen of grassier plants, particularly corn, as farming came to predominate human activity.
This idea would potentially supplant the Holocene epoch and replace it with the Anthropocene.
An end to atmospheric testing may have produced the declining bomb curve in these radioactive isotopes but it didn't stop geologic scale impacts, like the plutons produced by atomic blasts taking place underground until the 1990s.
These masses of shattered rock atop the radioactive melt core of a bomb test can be hundreds of meters across, and melt as much rock as a medium-size volcano.