Apt (; Provençal Occitan: At / Ate in both classical and Mistralian norms) is a commune in the Vaucluse department in the Provence-Alpes-Côte d'Azur region in southeastern France. It lies on the left bank of the Calavon, east of Avignon. It is the principal town of the Luberon mountains. The town is known for defining the Aptian age of the Early Cretaceous. Geography Apt lies north of Aix-en-Provence and the river Durance, in the valley of the river Calavon, (also called the Coulon), and at the foot of the north-facing slopes of the Luberon mountain. Geological age Apt is the etymological source of the Aptian, an age in the geologic timescale, a subdivision of the Early or Lower Cretaceous epoch or series and encompasses the time from 125.0 ± 1.0 Ma to 112.0 ± 1.0 Ma (million years ago), approximately. The original type locality is in the vicinity of Apt. The Aptian was introduced in scientific literature by French palaeontologist Alcide d'Orbigny in 1840. History Apt was at one time the chief town of the Vulgientes, a Gallic tribe; it was destroyed by the Romans about 125 BC and restored by Julius Caesar, who conferred upon it the title Apta Julia; it was much injured by the Lombards and the Saracens, but its fortifications were rebuilt by the counts of Provence. A traditional tale attributes the foundation of the bishopric of Apt to a saint named Auspicus, whom Pope Clement I sent and who died a martyr in 102; but the first documented evidence of its existence is in the acts of the Synod of Arles of 314, at which Apt was represented by a priest and an exorcist. Early 5th-century bishop Saint Castor of Apt is mentioned in contemporary liturgical documents and in a 419 letter of Pope Boniface I. The diocese appears in documents of the same century as a suffragan of Aix. As a result of the concordat of 15 July 1801 between Pope Pius VII and Napoleon Bonaparte, the territory of the diocese was incorporated by the bull Qui Christi Domini of 29 November of the same year mainly into the archdiocese of Avignon, with some parishes going instead to the diocese of Digne. No longer a residential bishopric, Apta (as it is called in Latin) is today listed by the Catholic Church as a titular see. Important manuscripts were found in Apt concerning music in