Environmental Engineering Reference
In-Depth Information
Plate 15.14
High-altitude, large-scale patterned ground on
Galdhøpiggen (2470 m), central Norway.
Photo: Ken Addison.
phenomena are not developed far enough to connect. Material inhomogeneity distorts the
circles, polygons and nets in those that do. In particular, gravity elongates the downslope
component on slopes as low as 2°-5° and stone stripes develop on slopes of 5°-25°. At
higher angles, however, the lateral component of thrusting is destroyed and stones
develop cross-slope stone-banked lobes or form more extensive screes. Cryoturbation
also exploits frost-susceptible and hydrological inhomogeneity in a more random manner.
Local variations in bulk density, porosity, water saturation and freezing rates generate
vertical separation and overturning in the active layer. Cryostatic pressure on a pocket of
saturated, unfrozen sediment may cause it to inject a plume into overlying material or
erupt at the surface whilst denser blocks may sink and intrude underlying material. In
extreme cases, entire unfrozen water bodies may collect in the substrate, especially in
valley floor peat or alluvium. They grow by cryostatic and/or artesian pressure as the
system starts to freeze. The water eventually forms a segregated ice lens which erupts as
a
palsa
up to 10 m high or grows into a perennial
pingo
10-80 m high (Plate 15.15). The
micromorphology of frost-disturbed ground creates edaphic
