The Path Between the Seas

by David McCullough

The worst troubles were in what was called the Cucaracha formation, composed chiefly of dark-green and reddish clays, lava mud flows, gravel, some shales. The first of the Cucaracha slides occurred on the eastern side of the Culebra Cut, where the uppermost layers of porous clay, layers overlying relatively impervious rock stratum, were from ten to forty feet thick. In the rainy season these clays became thoroughly saturated, slick and heavy, with a consistency of soap left overnight in water. But the saturation stopped at the underlying rock, and the build-up of water created a slippery zone along the whole plane of contact. If that plane happened to be tilted toward the Cut, then it was merely a matter of time until the clay began to move, by simple force of gravity, down into the Cut. Tremendous masses of the upper stratum would let go with all the effect of an avalanche, carrying with them whole sections of track, steam shovels, anything caught in the way.