Truncated spur

Bridal Veil Falls in Yosemite National Park. Either side of the waterfall are the flat vertical faces of the truncated spurs

Faulted southeastern side of Svyatoy Nos peninsula, Lake Baikal - active faulting shown by truncated spurs forming triangular facets

A truncated spur is a geologic formation created under specific conditions which result in a ridge of land with its end cut off. It occurs when ridge spurs, which were formed in a mountainous area by river action, have been (or are still being) truncated by the action of ice or active faulting.

Formation

Glaciation

Before glaciation, relatively immature rivers display a pattern of interlocking spurs. A valley glacier cannot avoid the interlocking spurs as a river can. As the valley glacier moves, abrasion and plucking erode the protruding tips of the spurs, leaving steep cliff-like truncated spurs. Hanging valleys are found in between truncated spurs as the they join the main glacial valley from the side. It is common for waterfalls to form from them, where they fall into the main valley.

Truncated spurs can be found in mountainous regions. The Mer de Glace, in the European Alps, is a valley through which a glacier currently flows. This is a geologically active process where the glacier continues to gradually erode the valley sides.

Active faulting

Truncated spurs are also formed by active faulting, especially normal faulting, producing characteristic triangular facets, sometimes known as a 'flat irons', at the mountain front. These landforms provide evidence for recent fault movement and are used in seismotectonic analysis.