SYMMETRY IN GLACIATED LANDSCAPE

In Southeast Alaskan landscape, adjacent drainage basins show drainage patterns that are approximate mirror images and corresponding portions are often equal in planar area.

These bilaterally symmetrical basins are separated by a ridge extending from a mountain mass and ridge which forms their upper boundary. Each basin is in turn divided by a similar ridge.

Twin drainage basins from different lithic terranes are similar in shape and size. The most perfectly equal and regular patterns face southwest and run at approximately right angles to local fault patterns and across terrane boundaries.

Examination of Prince of Wales Island (193km x 65km) reveals that the island is organized into four symmetrical basins, and shows that the phenomenon applies to the entire landscape. Faults establish the boundaries of these basins, but do not form elements of the symmetrical pattern.

A uniform symmetrical pattern indicates that one process is responsible for producing the entire pattern and that valleys were formed two and four at a time by some over-arching process. That process appears to be sub glacial erosion.

In a glacial channel, shear forces are symmetrically distributed and maxima are found near the edges of the channel. These maxima are at least twice that found in the center. If it takes a threshold amount of force to drive erosion, that threshold will first be met on the outside of the channel. The result: two channels are formed, with a ridge between them.

Observations of symmetrical drainage patterns in cirques, often miniatures of patterns that are hundreds of square kilometers larger, support the conclusion that this bi-laterally symmetrical drainage pattern is formed by erosion of bedrock under a lobe of flowing ice.