Summit elevations in the Lebanon Valley, part of the Great Valley, range from 440 to 720 feet above msl (mean sea level). This range cannot be accounted for adequately by the peneplain concept. Although accordant summits, the chief evidence for peneplains, occur over large areas, summits are not accordant between adjacent areas within the valley. The Lebanon Valley is underlain in the south by carbonate rocks and in the north by shale. The major stream valley in the carbonate area is now partly occupied by segments of two streams, but at one time it was the location of one major stream -the ancestral Quittapahilla Creek-which was beheaded by a tributary to Swatara Creek. Landforms of the Lebanon Valley are probably the result of erosion within two separate stream systems-Swatara and ancestral Quittapahilla creeks-in which streams and interfluvial areas were in a state of erosional equilibrium. The land surface in equilibrium with the ancestral Quittapahilla Creek lies at a higher elevation than adjacent land surfaces that were in equilibrium with Swatara Creek. The land surface on the carbonate rocks, which is in the ancestral Quittapahilla Creek system, lies at a lower elevation than shale within the same system, but it commonly lies at a higher elevation than shale in adjacent parts of the Swatara Creek system. Accordance of summits is the result of uniform erosion of uniform rocks in basins whose discharge points are at the same elevation. Lack of accordant summits on uniform rocks is the result of erosion in basins whose discharge points differ in elevation.