During much of the Cretaceous Period, from 144 to 65 million years ago, a large portion of Montana lay beneath a shallow inland sea called the Western Interior Seaway. The rims above Billings are made of Upper Cretaceous Eagle Sandstone, a cemented layer of fine-grained sand and silty mud deposited 70 to 80 million years ago as shoreline and offshore bars of that sea. Local sandstone has been used in foundations and exterior walls of buildings and homes, including the Parmly Billings Library, now the Western Heritage Center, and several buildings at Rock Mountain College. In 1926, a returning visitor to Billings, struck by the city's growth, remarked that the Rimrocks had not changed much.