LATITUDINAL VARIATION IN GLACIAL CIRQUE ALTITUDES

Glacial erosion at the base of cirque headwalls and the creation of threshold slopes above cirque floors may have a role in the “glacial buzzsaw” in limiting the altitude of mountain peaks. Since glacial extent and therefore glacial erosion rate depends on the equilibrium line altitude (ELA) of a region, the altitude of cirque formation should be a function of the ELA. Regional studies have shown that cirques form at an altitude midway between the modern and last glacial maximum (LGM) ELAs in some mountain ranges, but a global correlation has not yet been demonstrated. We examined the correlation between cirque altitudes and global ELA trends by compiling existing data for cirque locations and altitude in mountain ranges at varying latitudes including the: Washington Cascades, Swiss Alps, Uintas, the Northern Rockies in Glacier National Park, Bitterroots, Beaverheads, Lemhi Range, Tetons, and Lost River Range. We also used ArcGIS, 30 m to 100 m resolution digital elevation models, and geo-referenced 1:50,000 scale topographic maps to locate cirques and measure cirque altitude and relief in several additional ranges including the: Southern Alps, Andes, Rif and Atlas Mountains, Rwenzori, Alaska Range, and the Coast Range. For each range we calculated the average cirque altitude, relief, and latitude and compared the altitudes to the East Pacific modern and LGM ELAs. For the ranges studied, the average cirque altitudes generally fall between the modern Eastern Pacific and LGM ELAs. In the northern hemisphere, cirque altitude gradually increases towards the north from the LGM ELA to the modern ELA. Average cirque altitudes in the southern hemisphere are midway between the LGM and modern ELA. This evidence supports the hypothesis that cirque formation is dependent upon the ELA, and that cirques likely form during average, rather than extreme, glacial conditions. Average cirque relief for each mountain range varied from ~205 meters (in the Beaverhead Range, Idaho) to ~554 meters (in Glacier National Park), suggesting that peak heights are restricted to < 600 m above the cirque basins. The correlation between cirque altitude and ELA, along with the restricted window of relief, implies that cirque formation is a factor in limiting peak altitude in ranges that rise above the ELA.