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The Paris Basin
Between the Ardennes, the Vosges, the Massif Central, and the Massif Armoricain lie the sedimentary beds that make up the Paris Basin. Alternating beds of limestones, sands, and clays dip toward the central Paris Basin, their outcrops forming concentric patterns. Especially to the east, erosion has left the more resistant rocks, usually limestones, with a steep, outward-facing scarp edge and a gentler slope toward the centre of the basin. The central Paris Basin is filled by rocks from the Paleogene and Neogene periods, mostly limestones, that form the level plateaus of regions such as Beauce, Brie, Île-de-France, Valois, and Soissonnais. This area is mostly covered with windblown limon, which is the basis of an excellent loamy soil. The limestone levels overlap in sandwich formation. Eroded remnants of higher formations have been left behind as isolated hills called buttes, perhaps the most famous of which is in Paris—the Butte de Montmartre, on which is one of the city’s most famous districts. Sandy areas adjoining the limestone formations bear forests, such as the Forest of Fontainebleau, southwest of Paris. In the east, in the regions of Lorraine and Burgundy, are Triassic and Jurassic rocks; among the scarps the Moselle Hills are noted for their minette, low-grade iron ore. In the extreme southeast the Jurassic limestone Plateau de Langres forms the watershed between the Seine and Rhône-Saône river systems; it is crossed by major routes linking Paris with the south. The eastern basin includes the chalk country of Champagne and the Argonne massif. In the western part of the Paris Basin, scarps in the Jurassic and Cretaceous rocks of Normandy are not prominent. The chalk plateau is trenched by the lower Seine in a course marked by spectacular meanders and river cliffs. The plateau surfaces are frequently mantled by clay-with-flints and other residual deposits, producing heavy soils with much forest, grassland, and orchard cultivation. Farther north the wide chalk plateaus of Picardy and Artois are generally covered with limon, which provides for a rich agriculture; many stretches of magnificent white chalk cliffs line the English Channel coast.
The Flanders Plain
In the extreme north the French boundary includes a small part of the Anglo-Belgian basin. Coastal sand dunes protect the reclaimed marshes of French Flanders from invasion by the sea.
The Alsace Plain
East of the Paris Basin is the Alsace Plain, bordered by the Vosges on the west, the Saône basin on the southwest, the Jura Mountains on the south, the Rhine River on the east, and Germany on the north. The terrace and foothills bordering the Rhine are covered with soil-enriching limon. Alluvial fans, which are laid down by tributaries emerging from the Vosges, and much of the floodplain of the Rhine and its major tributary, the Ill River, are forested. The Sundgau region of the Alsace Plain, which lies between the Jura and the Ill River above Mulhouse, is another great alluvial fan overlaying impermeable clays, which hold up numerous lakes. The Rhine River and its tributaries continue to deposit thick sediments on the floodplain. The river is canalized, to the considerable detriment of the water table on both sides.
The Loire plains
Toward the southwest the Paris Basin opens on a group of plains that follow the Loire valley. The hills of this area, such as the limestone plateaus of the Touraine region and the crystalline plateaus of the Anjou and Vendée areas, are cut by the broad valleys of the Loire and its tributaries. The middle Loire valley, which varies in width from about 3 to 6 miles (about 5 to 10 km), is famous for its châteaus and its scenic beauty.
The Aquitaine Basin
The Loire countryside links with the Aquitaine Basin of southwestern France through the gap known as the Gate of Poitou. The Aquitaine Basin is much smaller than the Paris Basin, and, while it is bounded in the south by the Pyrenees, in the northeast it runs into the low foothills of the Massif Central. The slopes of both the Pyrenees and the Massif Central decline toward the central valley of the Garonne River. The Aquitaine Basin lacks the clearly marked concentric relief of the Paris Basin. In the north it has limestone and marl plateaus cut by the fertile river valleys emerging from the Massif Central. The southern low plateaus were mostly filled by a mass of rather ill-defined Paleogene and Neogene sands and gravel called the molasse, stripped off the rising Pyrenees. The foot of the central Pyrenees is marked by a remarkable series of confluent alluvial fans forming the Lannemezan Plateau. The Landes, an area lying between the Garonne and Adour rivers to the west, has a surface that consists of fine sand underlain by impermeable iron pan, or bedrock. The area, once covered by heath and marshes, is now reclaimed and planted with maritime pine. South of the wide, deep Gironde estuary, the Bay of Biscay coast is lined by enormous sand dunes, behind which are shallow lagoons.