Akousilaos from Argos, a hardly known ancient Greek writer, was a very important figure for ancient Greek History (Mythology included); he was one of the main ‘sources’ for most of the Historians and Mythographs that followed his time.
From the medieval Lexikon ‘Suda’ (which pumps information from ancient Sources, existing and stored at its writing time) we read:
«Akousilaos, the son of Kavas, from Argos, from the city of Kerkas which is near Aulis, very old historian. He wrote Genealogies from copper plates that were found by his father while digging somewhere inside his residential area»
So, Akousilaos (he lived in the 6th century BC) had been an ‘amateur archaeologist’ and publisher of the texts he found. Important remarks:
a) he found copper plates with engraved letters on them – full stories,
b) those plates, obviously, must have been engraved with the same language as the one Akousilaos understood, they simply belonged to an earlier era,
c) the stories written on the plates considered facts before the Cataclysm and a little after that (this is clear from the Sources we have, which are: Damascius, Platon, Sch. Theocritus, Philodimos, Apollodorus, Clemes, Didimus at Macrob., Pausanias, Sch. Apoll. Rod., Sch. Hesiodus, Sch. Homer, Sch. Nikandr., Sch. Pind., Strabon)
d) reasonable conclusion: the Greeks must have had writing even before the last* Cataclysm (whenever that was). The language and its writing should have hardly changed much since then, so that many centuries -or even millenia- afterwards, a posterior Greek, Akousilaos, found, read and published all those very old stories.
* we are referring to the ‘last Cataclysm’ because Greek Mythology suggests multiple cataclysmic events, smaller or bigger.