A geographic depression formed as a result of a continental collision.
When the movement of a tectonic plate forces it on top of another tectonic plate, isostatic forces depress the overriden plate. Where a continental plate overrides an oceanic plate, the result is a subduction zone.
When a continental plate overrides another continental plate, however, the effect is much more dramatic. Immense mountains are thrust up on the hinterland (on the overriding plate), and a wide area of the foreland (on the overridden plate in front of the advancement) is depressed. Usually the foreland is depressed enough to fall below sea level, and a small sea forms between the land masses. One example is the Adriatic Sea, which extends up into the Po River Valley in Italy.
Into this sea falls the sediment from the fast-eroding mountains of the hinterland. In the short term, over a few million years or so, the basin fills in with sediment from the eroding mountains forming wide, low plain. The best-known active foreland basin, the Ganges River Valley in India, is such a plain, having formed when the Indian Plate thrust itself under the Eurasian plate to form the Himalayas.
Eventually, the mountains are eroded away and only the sediments deposited in the basin remain. Isostatic rebound frequently forces the sediments, long since petrified, up into low mountains, a phenomenon known as "topography inversion". During eastern North America's three or four Paleozoic Era orogenies, a swath of continental crust was thrust up on top of the North American Plate. Himalaya-sized mountains formed in the hinterland, about where the East Coast is now, and the crust was depressed into foreland basins about where the Appalachian Mountains are
now. The sediments deposited in this basin now form the Catskill Mountains, the Allegheny Plateau, and the Cumberland Plateau.
To take a more mercenary tone, the swampy deltas that form in foreland basins trap masses of dead vegetation which, millions of years later, form into coal and oil deposits.
Since continental collisions have happened throughout geologic history, old foreland basins appear everywhere. Some examples include:
BASIN /`-._ HINTERLAND
FORELAND / `-._
______ delta / `--.____________
`-----._____ sea /
`-----.___ ______/ <- Movement
`---.wwwwwwwwwwwwww'=====/ <- of
`--.__/==========/ <- overriding
Overridden `--=======/ <- continental
`-==/___ <- plate
------._____ Plate `------._______