In the early twentieth century, small balls of stone from about 2500 BCE, identified by archaeologists as marbles, were found by excavation near Mohenjo-daro, in a site associated with the Indus Valley civilization. Marbles are often mentioned in Roman literature, as in Ovid's poem "Nux" (which mentions playing the game with walnuts), and there are many examples of marbles from excavations of sites associated with Chaldeans of Mesopotamia and ancient Egypt. They were commonly made of clay, stone or glass. Marbles arrived in Britain, imported from the Low Countries, during the medieval era.