Controls of tor formation, Cairngorm Mountains, Scotland

Abstract

Tors occur in many granitic landscapes and provide opportunities to better understand differential weathering. We assess tor formation in the Cairngorm Mountains, Scotland, by examining correlation of tor location and size with grain size and the spacing of steeply dipping joints. We infer a control on these relationships and explore its potential broader significance for differential weathering and tor formation. We also assess the relationship between the formation of subhorizontal joints in many tors and local topographic shape by evaluating principle surface curvatures from a digital elevation model of the Cairngorms. We then explore the implications of these joints for tor formation. We conclude that the Cairngorm tors have formed in kernels of relatively coarse grained granite. Tor volumes increase with grain size and the spacing of steeply dipping joints. We infer that the steeply dipping joints largely formed during pluton cooling and are more widely spaced in tor kernels because of slower cooling rates. Preferential tor formation in coarser granite with a wider joint spacing that is more easily grusified indicates that joint spacing is a dominant control on differential weathering. Sheet jointing is well developed in tors located on relatively high convex surfaces. This jointing formed after the gross topography of the Cairngorms was established and before tor emergence. The presence of closely spaced (tens of centimeters), subhorizontal sheeting joints in tors indicates that these tors, and similarly sheeted tors elsewhere, formed either after subaerial exposure of bedrock or have progressively emerged from a regolith only a few meters thick.

Inland notches: lithological characteristics and climatic implications of subaerial cavernous landforms in Israel

“…Recent studies in rock control upon geomorphology demonstrate that different erosion mechanisms may produce similar erosion features, but lithologically similar rocks might generate different landforms (Hall et al , ; Dorn et al , ). Thus, structural controls appear to be more critical than lithological or environmental controls in generating meso‐scale erosion products such as coastal rock outcrops (Cruslock et al , ) or tor formation (Goodfellow et al , ). Our findings suggest that inland notches form as a result of the interaction between specific lithological and climatic controls.…”

Quantifying the controls on potential soil production rates: a case study of the San Gabriel Mountains, California

“…Many studies have proposed a relationship between fracture density and bedrock weatherability on the basis that fractures provide additional surface area for chemical weathering and pathways for physical weathering agents to penetrate into the bedrock or intact regolith (e.g., Molnar, 2004;Molnar et al, 2007;Goodfellow et al, 2014;Roy et al, 2016a, b). The difference in erosion rates between the SGM and adjacent San Bernadino Mountains, for example, has been attributed in part to differences in fracture density between these ranges (Lifton and Chase, 1992;Spotila et al, 2002).…”

“…The Pelletier and Rasmussen (2009) model was a useful first step but clearly not all granites are the same. In particular, variations in mineralogy (Hahm et al, 2014) and bedrock fracture density (Goodfellow et al, 2014) can result in large variations in soil production rates in granites within the same climate.…”

Quantification of rock control in geomorphology

“…Tors also exists in the Cairngorm Mountains of Scotland (Goodfellow et al, 2014), where they have formed in spatially distinct kernels of granite that are relatively coarse grained and have a relatively wide spacing of steep joints compared with the surrounding granite from which regolith mantles have developed. Relative differences in grain size and joint spacing between the kernels and surrounding granite, rather than absolute values, govern whether a tor forms.…”