Glaciers perform, in many ways, like an excavator.  Although they can push weak material, like gravel, like a bulldozer blade, they are far more likely to lift material out of place, like a backhoe, or scratch it in place, like a ripper.  And, like a bulldozer, glaciers are poor at eroding rock unless it is already weakened.

Rock Failure:

      The first step in glacial erosion is rock failure. Rock failure can occur as the growth of pre-existing cracks or the formation of new cracks. Cracks form at weaknesses with in the rock, these weaknesses can be on the scale of large visible joints down to microscopic cleavage planes. Crack growth may be due to pressure release before glaciation, chemical weathering and volume change, or to the expansion of ice when water freezes. When water that has infiltrated into cracks freezes into ice it expands up to 10% in volume. The expansion forces the crack to widen, a process known as frost action.

     There are two main types of glacial erosion, the first one of which is plucking (analagous to a backhoe). As a glacier flows down hill there may be an obstruction which causes a high-pressure zone to form on the up-ice side and a low-pressure zone to form on the down-ice side. If the glacier is at the pressure melting point, basal ice melts in the high-pressure area and the resulting water infiltrates cracks present in the bedrock. Once this water has seeped through the bedrock and into the low- pressure area the water can refreeze. This frost action leads to the dislodging of rock fragments from the bed. This loose debris can also refreeze to the basal ice and be transported with glacier flow. Plucking, to be effective, requires a warm-based glacier. To see an animated model of plucking, click on figure 1.

      The other significant erosional process is abrasion. Plucked debris in basal ice (pic. 6) grinds into the bedrock, just like sandpaper across wood (figure 2). This grinding leaves long grooves in the bedrock called striations (if the sandpaper or ripper is coarse) or smooth polish (if the sandpaper is fine or the bulldozer is pulling a smoothing implement).  If the glacier is no longer present, striations can be used to determine the direction of ice flow. Abrasion can also be seen through the presence of chattermarks, arcuate pressure fractures.  Abrasion, too, requires a "warm" glacier bed.

Above photos courtesy of Dr. William Locke.

Above photos courtesy of Dr. Andrew Marcus.