In the Tertiary, a sea periodically extended from the foot of the Alps to the highlands of the Bavarian Forest. This sea was filled increasingly with weathering material from the Alps. The finest and youngest deposition layer of the Tertiary (upper fresh water molasse of the Miocene), the Flinz, now forms a water runoff, known as a soil horizon, which retains groundwater. For this reason springs emerge from the hillsides in the Isartal at several places in Munich, as can be observed at Spring Street, beneath the northern Hoch Street. These springs played an important role in the drinking water supply of the local population at least until the beginning of the 20th century.