1 Why do we study exfoliation joints?

Exfoliation joints are natural fractures limited to near the ground surface, i.e., they occur at a depth range important for rock extraction and engineering works. Exfoliation joints (also known as sheeting joints) follow the landscape surface at the time of their formation; the age of the associated landscape feature then provides a maximum age of exfoliation joints. While landscape forms can change through time, exfoliation joints preserve elements of former landscape morphologies by their undisturbed orientations.

Exfoliation fractures can play a key role in causing slope instabilities such as rockfalls and rockslides, increase the rates of rock mass weathering and erosion, but also, together with tectonic fractures, allow for economic extraction of dimension stones from a rock mass. Apart from this, we can use exfoliation joint properties (orientation, spacing, and fracture surface morphology) to constrain geologically recent and today's near-surface rock mass stresses, a rock mass state property that is essential for rock engineering. Furthermore, well-preserved fracture surface morphologies let us interpret fracture formation modes and processes.

2 What is special about the Grimsel region of the Central Alps?

The Grimsel region in Switzerland is well suited for analyzing the impact of erosional episodes, and accompanying stress changes, on exfoliation joint formation in granitic-gneissic rocks (Ziegler et al. 2013, 2014, 2016). Mapping above and below ground revealed that exfoliation joints are widespread and occur between valley bottoms and mountain crests within glacial (inner and hanging U-shaped trough valleys, glacial cirques, and steep mountain crests) and predominantly fluvial landforms (gently inclined linear slopes above the inner trough valleys, narrow inner-valley gorges, and steep V-shaped side gullies). Relating the mapped distribution and characteristics of exfoliation joints to identified erosional episodes and landscape features of known and inferred ages enabled us to distinguish four exfoliation joint generations in the Grimsel area, ranging from the lower Pleistocene (~1.5–1 Ma; generation 1) to the Late Glacial/Holocene (<0.02 Ma; generation 4). The most prominent and deepest exfoliation joint generation is associated with erosion of the inner glacial troughs of the upper Aar valley, which likely occurred during the 'mid-Pleistocene Revolution' (generation 2), followed by a joint generation with a minimum age dating to about the Last Glacial Maximum (generation 3).

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