 Walking from the upper falls to the base of the falls one passes through the layered Platteville Limestone Formation and a thin layer of Glenwood Shale . Just below 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 . 
