Libya is situated on the Mediterranean foreland of the African shield. Marine strata of Paleozoic, Mesozoic, and Cenozoic ages abound in northern Libya, but continental rocks of Paleozoic and Mesozoic ages predominate in southern Libya. Marine incursions in Ordovician, Silurian, Devonian, Carboniferous, late Cretaceous, and early Tertiary times reached far into the country, some crossing the southern border. Compressional folds are almost wholly absent, but uplift, subsidence, block faulting, and tilting have occurred, and angular and parallel unconformities are common. The major diastrophic disturbances include the Caledonian and Hercynian, as well as disturbances during late Cretaceous and Oligocene through Miocene or Recent times. The chief regional structures are the Gefara basin, Hamada basin, Gargaf arch, Marzuq basin, Tibesti-Haruj uplift, Kufra basin, Cyrenaican uplift, and Sirte embayment. Several large basalt flows of Cenozoic age are present. Sand and gravel conceal the bedrock in about a third of the country. In the Sirte embayment marine sedimentation, differential compaction, reef development, subsidence, and block faulting, beginning in late Cretaceous time, favored the development of source and reservoir rocks. Recoverable oil has been found chiefly in limestone and sandstone of early and late Cretaceous and Tertiary ages, and in knobs of probable early Paleozoic sandstone and in fractured granite. In the Hamada basin oil accumulations have been found in sandstones of Triassic age and of several Paleozoic systems. Most of the oil and gas discovered to date in Libya are in anticlinal structures, but several unconformities within the section probably influenced these accumulations; oil and gas may well await discovery in other types of traps.