Part 11

TOPPLING OF ROCK SLOPESToppling of columnar basalt recorded by Alexander von Humboldt during his exploration of the Rio Magdelana in Columbia in 1797.

Whenever the resultant weight of a block, W, projects beyond the downslope outside corner of a rectangular shaped blocks, toppling can result, as sketched here.Toppling usually occurs when low friction discontinuities dip between 50 and 70 degrees from horizontal. These can be joints, bedding, or foliation planes W

Flexural topplingFlexural toppling occurs when the length-to-width ratio of adjacent blocks causes the center of gravity to fall beyond the lower hinge point, or corner of the block.

Slope creep is often confused with toppling and vice versa. This shows classic slope creep in the Ozarks of Missouri, along iron stained joints in a precambrian rhyolite, leading up into the bedrock creep zone and overlying residuum.

Toppling is often easy to recognize by the textural contrast with the country rock. This shows the Clear Creek Toppling Complex developed in the Vishnu Schist, in the Granite Gorge of the Grand Canyon, near River Mile 84.

Rock topple along a forest road near Coos Bay, Oregon which killed two and injured another. Rain had preceded the event.

Blocky, jointed rock masses can be modeled using programs like UDEC, shown here. This shows an example of a toppling failure.