Pronival (protalus) ramparts: a review of forms, processes, diagnostic criteria and p alaeoenvi ronmental implications

Abstract

Most of the literature on pronival (protalus) ramparts deals with supposed fossil examples with very few studies devoted to active features and/or observed processes. Not surprisingly, this has led to circular reasoning and assumptions about typical rampart form, constituent material and genesis that have been shown to be inadequate or spurious from the few detailed investigations of actively forming ramparts so far carried out. Nevertheless, reliance continues to be placed on the characteristics of fossil features in the search for previously unidentified fossil examples. This article provides a critical review of rampart terminology, morphology and sedimentology, mechanisms, 'diagnostic' criteria, position in a continuum of talus-derived landforms and palaeoclimatic significance. It is suggested that the descriptor pronival is preferable to the widely used protalus, as the latter is misleadingly restrictive in terms of the known range of possible locations. Greater variability in terms of form and mechanisms of formation than most workers assume is indicated by recent studies of active features, with multiple as well as single ridges, fines and edge-worn as well as coarse angular clasts being found, and a variety of supranival and also subnival processes regarded as possibly contributing to rampart formation. It is concluded that only when further investigations of actively forming ramparts have been carried out, will it be possible to compile a reliable list of criteria for dist inguishing ramparts from moraines, protalus rock glaciers and other bedrock cliff-foot deposi tional forms. Correct identification of fossil ramparts may then lead to a better understanding of 1) their place in a continuum of talus-derived landforms; and 2) their potential as palaeoclimatic indicators.