Cirque form and development in Romania: Allometry and the buzzsaw hypothesis

摘要

A complete inventory of the 631 glacial cirques in Romania (and adjacent Ukraine) shows that they are scale-specific and develop allometrically, extending in length more rapidly than they deepen: shape changes with size. Horizontal dimensions are around 700 m, vertical around 300 m. On each quantitative measure they form a single, unimodal population with limited variation in size (on a logarithmic scale). Cirque axial gradients (15-33°) are appropriate to occupation by rotationally-flowing glaciers. Lake basins are more common (26%) on granitic rocks, compared with 10.6% of all cirques. Geology also affects vertical dimensions and gradients. The relation between subjective (five grades) and objective measures of cirque development is shown by an r~2 of 62% when Grade is predicted from maximum gradient, minimum gradient, and plan closure. Cirques larger in horizontal dimensions have better grades. Cirque floors cover around 28% of cirque map area. Larger cirques, and nested inner and outer cirques, are common in ranges that rose well above the snowline, and are associated with more extensive, more symmetrical gla-ciation and more cols. In size and shape Romanian cirques are similar to those in England and Wales. They are similar in some respects to those in part of the British Columbia Coast Mountains, except for smaller vertical dimensions. They may have developed only in the last few glaciations: mountain ranges can be ranked by degree of glacial modification and symmetry of glaciations. The 'buzzsaw hypothesis' is applicable only in the cores of the higher ranges: elsewhere, summit surfaces that are essentially preglacial survive.