More on quarrying

....... by which I mean glacial quarrying, the process by which chunks of bedrock are removed and entrained within overriding ice.

As a reminder, here is Prof Dave Evans's definition:

Quarrying (Prof David Evans)
Progress in Phys Geog 2004
 
Quarrying
Quarrying involves two separate processes: (1) the fracturing or crushing of bedrock
beneath the glacier;  and (2) the entrainment of this fractured or crushed rock.  Fracturing
of bedrock may take place where a glacier flowing over bedrock creates pressure
differences in the underlying rock, causing stress fields that may be sufficient to induce
rock fracture (Morland and Boulton, 1975; Morland and Morris, 1977). Fluctuations in
basal water pressure may also help to propagate bedrock fractures beneath a glacier
(Röthlisberger and Iken, 1981; Walder and Hallet, 1985; Iverson,1991a). Brepson(1979)
has successfully simulated the sliding of temperate ice over an obstacle in the
laboratory, and noted that large cavities form in the lee of obstacles, aiding quarrying.
Evacuation of rock fragments along joints in the bed is possible where localized basal
freezing occurs, for example as the result of the heat-pump effect proposed by Robin
(1976). Although Holmes (1944) originally argued that quarrying could occur beneath
both thick and thin ice, and outlined a theory based on pressure-controlled freezing of
meltwater in joints in bedrock, there is now general agreement that quarrying is
favoured beneath thin, fast-flowing ice (Hallet, 1996). Modelling studies indicate that
low effective basal pressures (0.1–1MPa) and high sliding velocities are the dominant
glaciological conditions required for quarrying because these conditions favour
extensive ice/bed separation (subglacial cavity formation)and also concentrate stresses
at points, such as the corners of bedrock ledges, where ice is in contact with the bed
(Iverson, 1991a; Hallet, 1996).

According to this theory, it may be that many of the features I have recently (again!) been looking at in the Stockholm Archipelago are very recent indeed -- formed just before deglaciation, which occurred here around 11,100 years ago.  Dave is picking up on a suggestion from many writers that when ice is very thick, movement on the bed might be negligible, with conditions effectively protecting rather than eroding the landscape beneath.  So for each glacial episode there might be two "erosional episodes" -- one at the outset of the glaciation, and the other at the end.  On the other hand there are other observations that suggest that thin ice may have a polar thermal regime on the bed, which means freezing-on and bedrock protection -- with erosion increased when the ice thickens because there may be a temperate thermal regime on the bed -- meaning high sliding velocities and considerable meltwater lubrication.  The truth is always more complicated than one would like it to be.........

Anyway, one thing that is blindingly obvious in the archipelago is that on the stoss (upglacier side) of all bedrock knolls and other roche moutonnees, abrasion and polishing features predominate, whereas on the lee side of all features plucking leaves rough and even jagged surfaces which show us where bedrock chunks have been dragged away.

Here are some photos of stoss-side slopes on RÖDLÖGA STORSKÄR:

Now here are some from the lee-sides or down-glacier sides of the same bedrock hillocks -- note how jagged surfaces, sharp edges and broken rock predominate.

 The only rounded or sub-rounded lumps of rock in environments like these are the glacial erratics dumped more or less at random during ice wastage.   It would be good to show some of these sites to certain British archaeologists, who would presumably (unless instructed otherwise) assume that all of them are Neolithic quarries littered with monoliths that were left behind by the builders of Stonehenge......