Talus, meaning “rock debris” or “slope of rock debris”

The word talus refers to rock debris at the base of a cliff, crag or valley shoulder. This word may also specify a slope covered with such rock debris [1,2]. In the latter case, one often speaks of a talus slope. A good example are the talus slopes below the south-facing cliffs of Castle Peak in the Sierra Nevada northwest of Truckee, California. The picture above shows Castle Peak seen from nearby Andesite Peak: concave talus slopes are skirting the cliffs keeping vegetation away from the upper mountain belt. The lower, less steep areas have some single conifers (survivors of rock slides). The forest begins where the slopes turn into a saddle and plateau topography, on the surface of which the impact of rolling rocks is becoming less powerful.   

Talus is created by weathering and fracturing of granite and other types of rock. Talus accumulates through periodic rockfall. F. J. Smiley, in 1915, briefly described the degrading process of Sierran mountain walls—while studying the Lake Tahoe region—and pointed out “the immense heaps of angular talus, which skirts the bases of Mt. Tallac, Maggie's Peaks and Castle Peak” [3]. 

A talus slope is always ready for a slide—triggered by an earthquake, an animal or a reckless mountaineer. Needless to say that a talus slope is dangerous terrain to walk across or to climb up or down on.

In addition to its meaning in topography and geology, the word talus means ankle or ankle bone in anatomy [1,2]. How the word and its meanings derived, is not completely clear. Old French, Latin or Celtic origins are typically mentioned. The plural form of talus is tali. Another word for talus is scree, probably from Old Norse skridha, meaning landslide [4].

Keywords: geology, etymology, synonyms.

References
[1] Merriam-Webster: talus [www.merriam-webster.com/dictionary/talus].
[2] The Free Dictionary: talus [www.thefreedictionary.com/talus]. 
[3] F. J. Smiley: The Alpine and Subalpine Vegetation of the Lake Tahoe Region. Botanical Gazette April 1915, 59 (4), pp. 265-286 [www.jstor.org/stable/2468057].
[4] The Free Dictionary: scree [www.thefreedictionary.com/scree].