Sheet Structure in Granites: Its Origin and Use as a Measure of Glacial Erosion in New England

Jahns, Richard H.

doi:10.1086/625130

Cited by 128 publications

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“…Typically, this conclusion is based on roche moutonnée-like landforms, where it can be demonstrated that abrasion on the top and stoss side was quantitatively less than erosion by plucking from the lee side (e.g., Jahns, 1943;Briner and Swanson, 1998).…”

Section: Discussionmentioning

confidence: 99%

“…However, this point has only been demonstrated for hard rocks such as granite (Jahns, 1943;Dühnforth et al, 2010) and gabbro (Briner and Swanson, 1998). We suggest that this rule cannot be applied as the general case but is, in fact, dependent on rock properties.…”

Section: Discussionmentioning

confidence: 99%

“…We note here that joints are amongst the most common geological structures on Earth. They are ubiquitous in well-stratified rocks in nonglaciated and glaciated regions alike and they are predominantly caused by non-glacial processes such as tectonic stresses, sedimentary loading, erosive unloading, and cooling (e.g., Jahns, 1943;Davis and Reynolds, 1996;Rogers and Engelder, 2004;Peacock, 2004). Consequently, we assume that the joints in the study areas are pre-glacial (e.g., Laubach and Diaz-Tushman, 2009).…”

Section: Bedrock Properties and Subglacial Erosion Mechanismsmentioning

confidence: 99%

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Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

Krabbendam

Glasser

2011

Geomorphology

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Subglacial erosion beneath glaciers occurs predominantly by abrasion and plucking, producing distinct erosional forms. The controls on the relative importance of abrasion vs. plucking are poorly understood. On the one hand, glacial conditions that favour or suppress cavity formation (ice velocity, ice thickness, and water pressure) are thought to favour plucking or abrasion, respectively.Conversely, bedrock properties are also known to control landforms, but this has rarely been analysed quantitatively. In this study we compare landforms and bedrock properties of sandstone and quartzite at the bed of a palaeo-ice stream near Ullapool in NW Scotland. The boundary between the rock types is at right angles to the westward palaeo-ice flow, and palaeoglacial conditions on both rock types were similar. We report quantitative parameters for bedrock properties (Schmidt hammer hardness and joint spacing) and use morphometric parameters to analyse the landforms. Torridon sandstone is soft but thick-bedded and with a wide joint spacing. Erosional bedforms include roche moutonnées with smoothed tops and concave stoss sides, whalebacks, and elongate p-forms, indicating a high proportion of abrasion over plucking. Cambrian quartzite is hard but thin-bedded with narrow joint spacing. Erosional landforms are angular to subangular with abundant plucked lee faces, suggesting a high proportion of plucking over abrasion. Hardness and joint spacing thus exert a strong control on subglacial erosional landforms and the mechanisms that formed them. Thus glacial conditions (ice velocity, ice thickness) can only be inferred from glacial erosional landforms if the effects of bedrock properties of the substrate are considered.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Bedrock Properties and Subglacial Erosion Mechanismsmentioning

confidence: 99%

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Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

Krabbendam

Glasser

2011

Geomorphology

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“…The most well known unloading fractures are curved exfoliation features, usually in non-layered, homogeneous materials such as the granites of the Sierra Nevada batholith (e.g., Bahat et al, 1999) and other locations (e.g., Jahns, 1943). These have classically been ascribed to removal of confining stress, although recent work provides an alternative explanation involving near-surface compressive stresses (Martel, 2011).…”

Section: Unloading Fracturesmentioning

confidence: 99%

Apparent glacially induced structural controls on limestone conduit development in Ohio Caverns, United States

Watts¹,

Sasowsky²

2016

IJS

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Abstract:Rock discontinuities such as bedding planes and joints are important controls on the form that caves take. We examined structural controls on the development of Ohio Caverns. The cave formed in Devonian limestone underlying a small bedrock knob (Mt. Tabor) within the Interior Lowland province, United States. The area has been overridden by continental glaciation multiple times. The bedrock is pervasively fractured, with many curved and wavy near-vertical fractures showing many different orientations. In the case of Ohio Caverns, it appears that the controlling fractures in map view may not be joints sensu stricto, but rather some combination of tensile and shear (mode-1 and mode-2) fractures, probably forming in the regime transition between tensile and shear fracturing. This is easy to envision in a situation with ice advancing over this topographic high, and would result in the curved fractures that are observed in many places in the cave. It can also explain the numerous fracture directions. However, not all fractures are conduit-significant. The cave initiated on or near a single bedding plane, and the cave passages exhibit strong keyhole or plus-sign cross sections. Passage and fracture orientations are inconsistent with regionally expected directions. It is likely that mechanical, hydraulic, and thermal stresses related to glaciation caused the fracturing in Mt. Tabor. The cave then developed on this template according to local hydrologic conditions. This presents a newly documented structural template sub-type for cave development.

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“…Whilst many exposed sheeting joints are evidently very recent, others are much older. Jahns (1943) and Martel (2006) note the apparent dissection of landscapes post-dating sheet joint formation. Antiquity is also indicated by preferential and thick weathering as illustrated in Fig.…”

Section: Development Of Sheeting Jointsmentioning

confidence: 99%

Sheeting Joints: Characterisation, Shear Strength and Engineering

et al. 2010

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Sheeting joints are extensive fractures that typically develop parallel to natural slopes. Embryonic sheeting joints initially constitute channels for water flow and then become the focus for weathering and sediment infill accompanied by progressive deterioration and dilation. Slabs of rock fail along them periodically because of their adverse orientation and long persistence. They are however rough and wavy and these characteristics contribute highly to their shear strength and improve their stability. This paper reviews several landslide case histories and on the basis of these provides guidelines for characterising sheeting joints and determining their shear strength. Engineering options for stabilising sheeting joints in natural and cut slope configurations are then examined with reference to case examples.

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Sheet Structure in Granites: Its Origin and Use as a Measure of Glacial Erosion in New England

Cited by 128 publications

References 0 publications

Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

Apparent glacially induced structural controls on limestone conduit development in Ohio Caverns, United States

Sheeting Joints: Characterisation, Shear Strength and Engineering

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