Standing Rock is not an isolated boulder but a remnant of the bluff that resisted erosion. This fine-grained sandstone may have formed when sand filled a sinkhole or joint within older dolomite, the main rock of these river bluffs, though like other mystery sandstones in the Ozarks its origin remains unclear. Another mystery is who marked flood levels on its face during the previous century. Standing Rock preserves water levels for at least seven floods: 1903, 1923, 1935, 1943, 1944, 1947, and 1998. The earliest mark, from a major June 1903 flood, records one of Kansas City's greatest natural disasters, while other notations are difficult to read. Although this trail lies more than a mile and a half from the river, it remains in the floodplain and vulnerable to the Missouri. The Missouri-Kansas-Texas Railroad, known as the MKT or Katy, chose this flood-prone corridor because a line with no grades and few curves approached railroad perfection, especially on its 165-mile Machens-to-Boonville section, but that choice brought constant danger from floods. During floods, railroad workers were mobilized for near-continuous hard duty to save the tracks. In 1935, despite the Dust Bowl drought on the Great Plains, heavy rains across the central Plains sent large volumes of water into every major river in Kansas and then into the Missouri through the Kansas River, covering virtually the entire MKT track along the river with 6 inches to 6 feet of water. In 1943, flooding followed a sudden snowmelt upriver, Katy trains detoured from St. Louis to Moberly, and fifty families were evacuated from Mokane with help from state guardsmen and volunteers. The 1947 flood was among the worst in more than a century: the railroad halted all livestock and perishable shipments between Sedalia and St. Charles, half of Mokane's 550 homes flooded, and the town's people were evacuated, while one account said ballast washed away like sand, rail was pulled from ties, and the ties floated downstream. Flooding's economic toll was a leading cause of the MKT's demise, and it ended operations in 1986 after a large flood badly damaged the tracks. Even so, the practice of marking great floods on Standing Rock continued, and someone returned to add the last major flood in Missouri, the flood of 1993.