Rocks

He found it in the herds of Eryx, and when the king refused to surrender it unless Hercules should beat him in a wrestling bout, Hercules beat him thrice, killed him in the wrestling, and taking the bull drove it with the rest of the herd to the Ionian Sea. But when he came to the creeks of the sea, Hera afflicted the cows with a gadfly, and they dispersed among the skirts of the mountains of Thrace. Hercules went in pursuit, and having caught some, drove them to the Hellespont; but the remainder were thenceforth wild. Having with difficulty collected the cows, Hercules blamed the river Strymon, and whereas it had been navigable before, he made it unnavigable by filling it with rocks; and he conveyed the kine and gave them to Eurystheus, who sacrificed them to Hera.


- Apollodorus, Library, Book 2, Chapter 5, Section 10