Early description of pro-talus ramparts

Sir,

Butler (1986) rightly pointed out that Daly (1912), early in the present century, drew attention to the distinctive alpine land form that Bryan (1934) subsequently named “protalus rampart”. However, Daly was not the first to describe and discuss the origin of these features. Cross and Howe (1905, p. 25) described pro-talus ramparts in the San Juan Mountains of Colorado which were forming at the front of snow banks, mainly at the base of north-facing cliffs, and referred to them as “snowbank accumulations”. In their words, the “snowbanks would form highways down which rocks loosened by frost would travel from the cliffs to the floors of the amphitheaters, and the effect would be to deposit waste at a greater distance from the cliffs than ordinary talus, and to protect a zone at the very foot of the cliffs from excessive talus accumulations."Howe (1909, p. 35–36, fig. 3) subsequently described them in more detail and illustrated their inferred origin by a diagram that is both clearer and more informative than the one which Daly (1912, fig. 41) later published. In this report, they were referred to as “snowbank deposits”. Although Daly referenced several U.S. Geological Survey publications in his memoir, he did not cite Howe’s observations; apparently that work was unavailable to him at the time of his writing. The Cross and Howe folio was published 7 years before Daly’s report appeared, but he may not have seen the brief description it contained.

In addition to Cross and Howe’s description of protalus ramparts, I have come across an even earlier discussion of these land forms in a paper that apparently escaped the attention of geologists working in the American west during the early years of this century. In describing the varied surficial deposits of the upper Indus River basin in the regions of Gilgit, Baltistan, and Ladakh, Drew (1873, p. 445) described different forms of talus along the valley walls and provided a description of what is clearly a pro-talus rampart:

“sometimes it happens that a talus of snow forms first, in much such position and form as the stone-heap itself might acquire; and then upon this snow-heap rolls down the loosened stuff, which therefore finds rest only at the foot, round the edge, of the snow-talus; the melting of this in summer leaves a heap of stones which may be of considerable height, though it is not very likely to increase by additions in successive seasons. Such circumstances as these should be borne in mind when one meets with isolated heaps, not far from the mountain-side, which might otherwise be taken for moraine-heaps … the heap at the foot of the snow talus is not unlikely to take the form of part of a ring abutting at its ends against the mountain, and thus enclosing a hollow …”.

This description, which antedates Daly’s by more than a third of a century, may constitute the earliest mention of these alpine land forms by a geologist.

Butler (1986) argued that Daly’s term “winter-talus ridge” takes precedence over the term “protalus rampart” later introduced by Bryan. One could argue equally well that Cross and Howe’s term “snowbank accumulation” (or Howe’s subsequent “snowbank deposit”) should take precedence over Daly’s term. However, I find neither of these terms very satisfactory, for they are imprecise and ambiguous. Daly’s term can also be faulted, for it implies that the talus ridge forms in winter. Although this may be true in some cases, I know of no definitive studies showing that sliderock production and accumulation at the toe of a snow bank occurs exclusively, or even predominantly, during the winter season. Some studies have shown that release of rock debris from mountain slopes reaches maximum frequency during mid- to late spring (May–June) when rapid thawing of frost-rived cliff faces takes place (Rapp, 1960); in polar latitudes, such activity may peak during the summer (e.g. Åkerman, 1984). Rock debris generated during the height of the accumulation season tends to become buried within the growing snow bank and would be unlikely to slide or roll down the unconsolidated snow surface to its toe. Bryan’s term “protalus rampart” avoids these problems, for it is a descriptive, non-genetic designation. Until more is learned about how and when these land forms develop under different geographic conditions, the widely used term proposed by Bryan remains a viable and preferred one.