Sandstone geomorphology – Recent advances

Highlights

• Comprehensive review of recent publications on sandstone relief at variety of scales

• New look at weathering patterns on rock faces – stress and hydraulic field important

• Subterranean processes are fundamental to many sandstone landscapes.

• New conceptual models of sandstone canyon, escarpment and tabular relief evolution

• Opportunities for geomorphometry in sandstone terrains outlined

Abstract

This paper provides a review of recent publications dealing with different issues of sandstone geomorphology, which was long an under-researched theme. Nevertheless, considerable advances have been made recently regarding the origin of various characteristic landforms and landscapes developed in sandstones. The structure of this review reflects the diversity of spatial scales, so that it begins with minor landforms and moves towards regional landscapes, considering both the form and the process. It first focuses on the origin and evolution of microforms on rock surfaces, especially honeycombs and arcades, which have been demonstrated to reflect the distribution of stress within the rock mass and the hydraulic field. A distinction is made between ‘organizing’ and ‘decay’ factors, with the latter varying depending on local circumstances, whereas the overarching role of the former explains equifinality. Then the concept of negative feedback between stress and erosion is reviewed, with reference to medium-scale landforms, mainly pedestal rocks and rock arches. In the next part new evidence and ideas about the role of subsurface processes in shaping medium-size and major landforms is presented, with emphasis on chemical decay (arenization), mechanical erosion and sapping. Larger landforms discussed are rock cities and ruiniform relief, canyons and mesas. It is argued that subsurface processes play an important part in the evolution of rock cities and boulder-filled canyons, whereas recent work on residual hills shows that plateau ➔ mesa ➔ butte ➔ pinnacle trajectory is but one possible pathway of geomorphic evolution of tablelands. In the following part a new concept of in situ disintegration of sandstone-capped escarpments is presented as complementary to the widely accepted model emphasizing large scale, often catastrophic mass movements. Finally, recent attempts to use geomorphometry tools to characterize rugged sandstone relief are presented. The paper is closed with an outline of challenges for the future.

Introduction

Sandstones are ubiquitous rocks at the Earth surface, commonly occurring in thick and laterally extensive successions, with stacked packages which may or may not be separated by finer-grained sediments (mudstones, siltstones, clays) or coarser conglomerates. Many sandstones are dominated by quartz in their skeletons, which affords higher resistance against weathering, but the composition of matrix is also very important, controlling the efficacy of many exogenous processes affecting sandstone bodies. Thus, whereas widespread occurrence favours the origin of a distinctive type of morphology on sandstones, different from that developed on other rocks in the vicinity, lithological and structural diversity within sandstone packages themselves is reflected in the range of medium-scale and minor landforms, including some which hardly have parallels elsewhere, on other lithologies. Some of the most striking erosional sceneries on Earth are supported by sandstones, solely or dominating over other rock types, and protected at international (UNESCO World Heritage, UNESCO Global Geoparks) and national level (national parks, state parks, nature reserves). Appreciation of sandstone scenery provides a background to their geoconservation, which in turn requires better understanding of process-response systems in sandstone terrains, at the variety of spatial scales, from microscale of an individual outcrop to region-wide landform patterns.

For many years, sandstone geomorphology was in relative neglect and even though various studies of weathering and erosion were conducted in sandstone areas, they did not build up to a better global recognition of the significance of sandstone landscapes. Among these studies particularly notable were an early overview of sandstone landforms by Mainguet (1972), which sadly received insufficient international recognition, and the development of sapping concept in the sandstone-dominated areas of southwestern United States – Laity and Malin (1985) and Howard and Kochel (1988). This limited interest remained in stark contrast to huge scientific interest in limestone (karst) areas and less obvious, but still significant focus on granite landforms and landscapes. Nevertheless, nearly 30 years ago Young and Young (1992) attempted a first scholarly synthesis of sandstone geomorphology, reviewing work done until that date and identifying a number of key themes to be investigated further in the future. This work was significantly updated after more than 15 years later (Young et al., 2009) and the resultant book remains a benchmark and starting point to any studies of geomorphology of sandstone areas. Among ‘hot topics’ addressed by Young et al. (2009) were slope forms in sandstones, which vary from vertical cliffs to ideally curved shapes, and how they related to the stress field patterns, the role of solution processes in sandstones, climate–landform relationships and lithological and structural diversity within the sandstone-dominated packages. A few years later, Young and Wray (2015) reiterated some of their views, emphasizing rock control as a necessary framework to better understand the astounding geodiversity of sandstone terrains.

Although research focused on sandstone landforms has not become a priority theme in the contemporary geomorphology, the last decade, and the last few years especially, have witnessed the appearance of both a number of innovative concepts as well as thoroughly documented examples of landforms developed in sandstones, including those from areas poorly covered in internationally accessible literature before (e.g., Central Europe, China, some South American countries). They address various spatial scales, from minor but nonetheless highly characteristic ornamentations of sandstone surfaces (arcades, honeycombs) through medium-scale landforms (e.g., rock arches, canyons) to regional landscapes (e.g., escarpments, large tracts of ruiniform relief). These studies also show a wide range of methods used, with modern technologies and sophisticated approaches increasingly applied, facilitated by the availability of high-resolution topographic data which allow for objective presentation of sandstone relief. Finally, quite a significant number of regional studies appeared in the World Geomorphological Landscapes book series and whereas the format of the series dictates a descriptive, review-type presentation, these studies are filling important geographical gaps (Table 1).

This paper is intended as a review of these recent advances and is generally focused on studies emerging in the last two decades and covering themes, which are not only innovative in the field of sandstone geomorphology, but are also believed relevant to the progress of geomorphology as a whole. The logic of presentation is to upscale from minor landforms to regional landscapes, considering both the form and the process. A special section is devoted to subsurface processes which have been shown to produce, directly or indirectly, a range of spectacular landforms in sandstone areas. Subsurface processes are rarely discussed in contexts other than carbonate and sulphate karst, but for sandstones they appear of key importance and help to understand the origin and evolution of both medium- and large-scale landforms such as ruiniform relief and sandstone-capped escarpments. Finally, a short section considers the role that geomorphometry can play in sandstone geomorphology studies, whereas challenges for the future are outlined at the very end. Unfortunately, because of length limitations several themes could not have been covered in this review (e.g., sandstone as building stone, landform-soil interactions in sandstone areas, non-karstic caves in sandstones) – their exclusion does not imply that they are of lesser importance.