the peninsula of Mettnau (the Markelfinger Winkel), and the Zeller See, south of Radolfzell and to the northwest of the Reichenau island, and the mainly Swiss Rheinsee (lit.: "Rhine Lake") – not to be mistaken for the Seerhein at its start – to the south of the island and with its southwestern arm leading to its effluent in Stein am Rhein. The river water of the regulated Alpine Rhine flows into the lake in the southeast near Bregenz, Austria, then through the Upper Lake Constance hardly targeting the Überlinger See, into the Seerhein in the town of Konstanz, then through the Rheinsee virtually without feeding both German parts of the Lower Lake, and finally feeds the start of the High Rhine in Swiss town Stein am Rhein. The lake itself is an important drinking water source for southwestern Germany. The culminating point of the lake's drainage basin is the Swiss peak Piz Russein of the Tödi massif of the Glarus Alps at above sea level. It starts with the creek Aua da Russein (lit.: "Water of the Russein"). Car ferries link Romanshorn, Switzerland, to Friedrichshafen, and Konstanz to Meersburg, all in Germany. History Lake Constance was formed by the Rhine Glacier during the ice age and is a Zungenbecken or tongue lake. After the end of the last glacial period, about 10,000 years ago, the Obersee and Untersee still formed a single lake. The downward erosion of the High Rhine caused the lake level to gradually sink and a sill, the Konstanzer Schwelle, to emerge. The Rhine, the Bregenzer Ach, and the Dornbirner Ach carry sediments from the Alps to the lake, thus gradually decreasing the depth and reducing the extension of the lake in the southeast. In antiquity the two lakes had different names; later, for reasons which are unknown, they came to have the same name. In the 19th century, there were five different local time zones around Lake Constance. Constance, belonging to the Grand Duchy of Baden, adhered to Karlsruhe time, Friedrichshafen used the time of the Duchy of Württemberg, in Lindau, the Bavarian Munich time was observed, and Bregenz used Prague time, while the Swiss shore used Berne time. One would have needed to travel only to visit five time zones. Given the amount of trade and traffic over Lake Constance, this led to serious confusion. Public clocks in harbors used three different clock faces, depending