It depends on which valley you're talking about, but most were created by erosion. Either that or they're the space between two hills/mountains.
In the Valley was created in 1890.
In a Valley by the Sea was created on 2007-10-04.
Valley of the Shadows was created in 1993.
The Valley of the Lost was created in 2002.
Valley Cemetery was created in 1840.
The Lackawanna Valley was created in 1855.
The Valley of Bones was created in 1964.
Valley Flyer was created in 1939.
The Valley School was created in 1978.
Lights from the Valley was created in 1978.
Valley Hotel was created in 1815.
How Green Was My Valley was created in 1939.