The Upper Rhine plain, a rift valley some 30-40 km/20-25 mi wide, is bounded on the east by the Black Forest, the Kraichgau and the Odenwald, on the west by the Vosges, the Haardt and the uplands of the northern Palatinate. Its lose soil makes it a fertile fruit- and vine-growing region (Markgräflerland, Kaiserstuhl, Ortenau, Deutsche Weinstrasse, Bergstrasse).
In its middle course the Rhine flows between the Rheingau (on right) and Rheinhessen (on left) for some 100 km/60 mi. Both of these areas lie within the western Mainz basin, which forms the northern termination of the trough-like depression of the Upper Rhine plain and, like it, was the result of a rift in Tertiary times. The Rheingau and the Rheinhessen uplands were at one stage submerged by water and were separated from one another only in geologically recent times. How far the water extended at one time is shown by the interesting fossil-rich deposits which can be seen in sand and marl pits at Gau-Algesheim, Sprendlingen, Messel and Weinheim.
At Bingen the river, which at Mainz had come up against the wall of the Taunus and made a sharp turn westward, changes its course again and flows through the Rhenish Uplands against the grain of the rock, cutting across the hard quartzites of the Hunsrück and the Taunus. In this resistant rock it is confined to a narrow gorge-like valley. The hills rose slowly from the Tertiary onwards, while the Rhine cut its way in stages into a pre-existing trough, creating a terraced landscape pattern. As a result boulder clay deposited by the Rhine is found at varying altitudes.
The passage through the Rhenish Uplands, with its changes of gradient, also creates difficulties of navigation - at the Binger Loch (Bingen Hole), the legendary Loreley Rock and St Goar. In the more open loess basins between these places there is room for prosperous settlements, for the growing of vines and fruit. These variations, together with the castles crowning the steep hills on either side and the islands in the river, produce an ever-changing pattern of scenery.
Below Koblenz, where the Mosel flows into the Rhine at Deutsches Eck, the valley opens out into the little Neuwied basin, where there has been a vigorous development of industry. Shortly before the river enters the Lower Rhine plain it passes on the right an outlier of the Westerwald, the distinctively shaped Siebengebirge (Seven Hills), forming a striking landmark at the lower end of the Middle Rhine.
The river then enters the Cologne or Lower Rhine lowlands, a gently undulating region. The Lower Rhine really begins at Duisburg. Then at Elten, below a hill crowned by a monastery, the Rhine crosses the frontier into the Netherlands.