Excursus on gravity gliding and gravity spreading

The terms ‘gravity gliding’ and ‘gravity spreading’ have long been used to describe deformation driven by gravity alone. However, the traditional definitions of these terms cannot be applied unambiguously in many situations. The primary difficulties arise because rocks are not ideally rigid, detachment surfaces may not be planar, substrates may be deformable, and rock bodies do not deform in isolation. The term ‘gravity spreading’ is still useful if it is simply defined as gravity-driven lateral extension and vertical contraction, regardless of basal slope and coherence of the body. I suggest that the term ‘gravity gliding’ should be used rarely, and only if the defining characteristics are clearly stated and understood. In most cases, more detailed descriptions should be used instead of, or in addition to, either of these terms to capture the behavior of rock masses deforming under gravity.