of the falls one passes through the layered Platteville Limestone Formation and a thin layer of Glenwood Shale. At the base of the falls one enters the level of the Saint Peter Sandstone Formation of pure white quartz sandstone. The Minnesota St. Peter Sandstone is "famous the world over as the example of a well-rounded, well-sorted, pure quartz sand. It is the Ivory Snow of sediments, because it is close to 99.44% pure". This sand was first deposited as beach sand, probably eroded from earlier Cambrian sandstones, along the shores of the Ordovician sea. At places in the park, especially closer to the Mississippi River, one can see the Glenwood Shale Formation. This thin layer of grey-green rock was probably deposited in deeper-water offshore from the beaches, and now has been exposed by erosion. Together, the three formations that are visible as one walks from the falls to the river represent a sequence of sea-level rise which occurred during the Ordovician period. A fourth geological layer that lies above the Platteville limestone, Decorah shale, can also be found in the park. The Decorah shale formation is so packed with fossils that it has been called "fossil hash". It has been washed away directly at the falls area but can be found above the Plattville limestone in the exposed river bank below Fort Snelling. Historical buildings Minnehaha Depot A small train station officially named Minnehaha Depot but also known as "the Princess Depot" was built around 1875; it was a stop on the Milwaukee Road railroad and provided easy access to the park from Fort Snelling and Minneapolis. The depot handled as many as 39 round trips per day. The interior is complete and well-preserved, containing an iron heating stove, waiting room benches and a ticket window. In 1964, the title was transferred to the Minnesota Historical Society. The Minnesota Transportation Museum assisted in the restoration of the building, and today provides historic interpretation. The depot is open in the summer on Sundays from 1:30 to 4:30. The 50th Street / Minnehaha Park light rail station of the METRO Blue Line currently serves the park. Longfellow House Despite its name, the Longfellow House was never home to the American poet Henry Wadsworth Longfellow; it was instead the home of Robert F. Jones, a Minneapolis philanthropist and entrepreneur. Built in 1907, the house's facade was designed to resemble Longfellow's home in Cambridge,