Glacial Erratic - Formation of Erratics

Formation of Erratics

The term "erratic" is commonly used to refer to erratic blocks, which Geikie describes as: "large masses of rock, often as big as a house, that have been transported by glacier-ice, and have been lodged in a prominent position in the glacier valleys or have been scattered over hills and plains. And examination of their mineralogical character leads the identification of their sources…". In geology, an erratic is material moved by geologic forces from one location to another, usually by a glacier.

Erratics are formed by glacial ice erosion resulting from the movement of ice. Glaciers erode by multiple processes: abrasion/scouring, plucking, ice thrusting and glacially-induced spalling. Glaciers crack pieces of bedrock off in the process of plucking, producing the larger erratics. In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till. In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier. Glacially-induced spalling occurs when ice lens formation with the rocks below the glacier spall off layers of rock, providing smaller debris which is ground into the glacial basal material to become till.

Evidence supports another option for creation of erratics as well, rock avalanches onto the upper surface of the glacier (supraglacial). Rock avalanche–supraglacial transport occurs when the glacier undercuts a rock face, which fails by avalanche onto the upper surface of the glacier. The characteristics of rock avalanche–supraglacial transport includes:

• Monolithologic composition - a cluster of boulders of similar composition are frequently found in close proximity. Commingling of the multiple lithologies normally present throughout the glaciated basin, has not occurred.
• Angularity - the supraglacially transported rocks tend to be rough and irregular, with no sign of subglacial abrasion. The sides of boulders are roughly planar, suggesting that some surfaces may be original fracture planes.
• Great size - the size distribution of the boulders tends to be skewed toward larger boulders than those produced subglacially.
• Surficial positioning of the boulders - the boulders are positioned on the surface of glacial deposits, as opposed to partially or totally buried.
• Restricted areal extents - the boulder fields tend to have limited areal extent; the boulders cluster together, consistent with the boulders landing on the surface of the glacier and subsequently deposited on top of the glacial drift.
• Orientations - the boulders may be close enough that original fracture planes can be matched.
• Locations of the boulder trains - the boulders appear in rows, trains or clusters along the lateral moraines as opposed to being located on the terminal moraine or in the general glacial field.