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Geography

The western boundary of the plains is usually well-defined by the abrupt ascent of the mountains. The eastern boundary of the plains (in the United States) is more climatic than topographic. The line of 20 in (510 mm) of annual rainfall trends a little east of northward near the 97th meridian. If a boundary must be drawn where nature presents only a gradual transition, this rainfall line may be taken to divide the drier plains from the moister prairies. However, in Canada the eastern boundary of the plains is well defined by the presence of the Canadian Shield to the northeast.