This section of the dam had been attached to the section that remained upright, known as the tombstone. This section broke off, skidded out feet-first and landed on its back in front of (immediately south of) the tombstone.

Photo by Hetta Laurena Carter a few weeks after the St. Francis Dam broke.

Further reading: Failing St. Francis: Water Pressure or Political Pressure?

Construction on the 600-foot-long, 185-foot-high St. Francis Dam started in August 1924. With a 12.5-billion-gallon capacity, the reservoir began to fill with water on March 1, 1926. It was completed two months later.

At 11:57:30 p.m. on March 12, 1928, the dam failed, sending a 180-foot-high wall of water crashing down San Francisquito Canyon. An estimated 431 people lay dead by the time the floodwaters reached the Pacific Ocean south of Ventura 5½ hours later.

It was the second-worst disaster in California history, after the great San Francisco earthquake and fire of 1906, in terms of lives lost — and America's worst civil engineering failure of the 20th Century.