Limestone and dolomite of the Mississippian epoch underlies the area and much of the city is akarst area, with numerous sinkholes and caves, although most of the caves have been sealed shut; many springs are visible along the riverfront. Significant deposits of coal, brick clay, andmillerite ore were once mined in the city, and the predominant surface rock, the St. Louis Limestone, is used as dimension stone and rubble for construction.
The St. Louis Geologic fault is exposed along the bluffs and was the source of several historic minor earthquakes; it is part of the St. Louis Anticline which has some petroleum and natural gasdeposits outside of the city. St. Louis is also just north of the New Madrid Seismic Zone which in 1811-12 produced a series of earthquakes that are the largest known in the contiguous United States. Seismologists estimate 90% probability of a magnitude 6.0 earthquake by 2040 and 7-10% probability of a magnitude 8.0,[2] such tremors could create significant damage across a large region of the central U.S. including St. Louis.
Near the southern boundary of the City of St. Louis (separating it from St. Louis County) is the River des Peres, virtually the only river or stream within the city limits that is not entirely underground. Most of River des Peres was either channelized or put underground in the 1920s and early 1930s. The lower section of the river was the site of some of the worst flooding of the Great Flood of 1993. Near the central, western boundary of the city is Forest Park, site of the 1904 World's fair, the Louisiana Purchase Exposition of 1904, and the 1904 Summer Olympics, the first Olympic Games held in North America. At the time, St. Louis was the fourth most populous city in the United States.
The Missouri River forms the northern border of St. Louis County, exclusive of a few areas where the river has changed its course. TheMeramec River forms most of its southern border. To the east is the City and the Mississippi River. According to the United States Census Bureau, the city has a total area of 171.3 km² (66.2 mi²). 160.4 km² (61.9 mi²) of it is land and 11.0 km² (4.2 mi² or 6.39%) of it is water.
