CONTROL OF GLACIAL QUARRYING BY BEDROCK JOINTS

The formation of overdeepened valleys and basins in glaciated alpine environments is the result of enhanced subglacial erosion that is likely dominated by quarrying. This process is thought to depend on slow growth of preexisting, isolated cracks near lee surfaces of bedrock bumps. However, most hard glacier beds contain tectonic joints and other macroscopic cracks that are commonly dense, pervasive, and aligned. Thus, these pre-existing weaknesses may be a first order control on quarrying, such that it occurs largely along pre-existing joints rather than by growth of isolated cracks as currently modeled.

To study the importance of pre-existing weaknesses in quarrying, we measured and compared the orientations of bedding planes and tectonic joints to those of quarried bedrock surfaces in the deglaciated forefields of eight valley glaciers in Alberta and Switzerland. The results demonstrate a strong correlation between orientations of joints and quarried surfaces that is independent of the ice-flow direction as recorded by striations. This strong correlation persists across various bedrock types, including limestone, granite, and schist. Thus, pervasive pre-existing weaknesses in the rock appear to be the primary control on quarrying. The implication of these observations is that quarrying may require either minor or zero crack growth to isolate bedrock blocks from surrounding rock and thereby prepare them for removal by the glacier. Current mechanical models likely overestimate the importance of slow crack growth in controlling rates of quarrying.

We propose a new conceptual model of quarrying that idealizes the bedrock as a series of blocks separated by discontinuous preglacial joints containing intact rock bridges. Bridges concentrate stress differences caused by normal and shear forces acting at the rock surface. Failure of bridges is caused by slow crack growth enhanced by water pressure fluctuations. To lend credibility to this model, we show field evidence of failed rock bridges on quarried surfaces and of rib marks on plumose structures that we interpret as arrest fracture fronts due to transient subglacial water-pressure fluctuations.