The very first layers of soil were dropped onto the Wallis Annenberg Wildlife Crossing on Monday morning, marking another major milestone for what will become the world’s largest bridge of its kind.

There’s a unique layer of soil called the Pittsburgh red bed and when it gets wet it starts to slide. “The Pittsburgh red beds are high in clay content.

CBS News Los Angeles The Wallis Annenberg Wildlife Crossing over the 101 Freeway in Agoura Hills got its first layer of soil on Monday. The bridge, made with 26 million pounds of concrete, is ...