Final answer:
The Interior Plains are composed predominantly of sedimentary rocks, primarily undeformed Paleozoic marine rocks like limestones and shales shaped by glaciers and rivers.
Step-by-step explanation:
The Interior Plains are primarily made up of sedimentary rocks. These rocks originated from the eroded material of the Rocky Mountains and are relatively young, dating back to the Mesozoic and Cenozoic eras. The sedimentary rocks of the Interior Plains include layers of undeformed Paleozoic marine rocks, such as limestones and shales, which have been carved by the action of glaciers and rivers over time. Intriguing geologic features, such as the spectacular cave systems found in the marine limestones of Mammoth Cave, Kentucky, are a testament to the geological history of the Interior Plains.