What historical sources can be found in the earth?

Over time, all artifacts and confirmation of some historical events turn out to be buried underground. The soil above such “treasures” is accumulated very slowly, for thousands or millions of years, due to human waste or slowly decaying biomasses.
Historical sources are divided into 3 types: material (such as dinosaur bones or architecture), written (birch bark letters) and oral (fairy tales). From this underground, obviously, only the first 2 sources can be found.



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