Click here to go to part one. HE earth comes unglued for a variety of reasons, but there is usually a single triggering event. Thus there have also been spectacular slides in Iowa and in the placid river valleys of Quebec. Pittsburgh and Cincinnati, constructed on loose sediments cut by river systems, incur some of the highest landslide costs in the United States. Aftermath of the 1995 La Conchita landslide, in southern California.
Mature soils settle out into homogenous layers, called horizons.
If colluvium stays still long enough, it may form horizons too. But on unstable hillsides it remains young and restless, a poorly sorted conglomeration of silt, sand, pebbles, and rocks. Colluvium covers most of the ground surface in humid temperate zones and semi-arid mountainous areas. It is ubiquitous in the western United States and the Appalachian Range, from Maine to Alabama. Colluvium appears to be just sitting there, growing trees, but over hundreds and thousands of years it is on the move, and the direction of its movement is always downhill.