The section of the face of the St. Francis Dam known as Block 5 lies at the foot of the "tombstone" (the center section of the dam that remained standing).

Original 5¼x3-inch glossy 1928 photo (photographer unknown). Looking east, the slippery Pelona schist is visible in the background, and a pile of schist can be seen on top of Block 5. Note the crack along the block, which they have patched (sort of) with oakum. A soluble fiber, oakum was typically used to plug leaks — all dams leak — but it's supposed to be used on the inside of the dam so the water can push against it and create a seal. Here, they've used it on the outside of the outward-curving dam. What do you suppose happens when water pushes on it from behind?

Compare to this photo.

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 411 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.