Several of the islands have been joined to the mainland by alluvial deposits. Herodotus says that half of the islands had been already united to the mainland in his time (ii. 10); and Thucydides expected that this would be the case with all of them before long, since they lay so close together as to be easily connected by the alluvium brought down by the Achelous River (ii. 102.). This expectation, however, has not been fulfilled, which Pausanias attributed (viii. 24. § 11) to the Achelous bringing down less alluvium in consequence of the uncultivated condition of Aetolia; but there can be little doubt that it is owing to the increasing depth of the sea, which prevents any perceptible progress being made.