GLACIAL AND PERIGLACIAL HIGH-MOUNTAIN DYN AMICS IN THE NORTHERN HENRY-MOUNTAINS (UTAH, USA) AND THEIR INFLUENCE ON THEIR ARID PIEDMONTS

The pediments in the northern piedmont of the Henry Mts. were formed d uring the Quaternary. The Blue Valley Bench-Pediment, located 200 m ab ove the Fremont River, represents the oldest relic of the Quaternary r elief development. The relics of the debris cover on the lower pedimen t levels are interlocked with the fluviatile deposits of the Fremont R iver, which appear as terraces 120 m, 90 m and 60-70 m above the recen t valley bottom. They are dated as early respectively middle Quaternar y. At that time a change between sheet erosion under arid conditions a nd debris covering in the following cold periods occurred in the whole piedmont irrespective of the lithological differences. An increased f low-off activity during glacial periods is also indicated by basaltic boulders in the Fremont River terraces. Near the mountain front perigl acial debris was deposited at the same time, and rock glaciers also ad vanced. Not later than during the Wisconsin the courses of the main st reams had been determined by these periglacial debris ridges. Then the river deposits were accumulated as isolated alluvial fans. The distal part of the piedmont was however formed under quite different morphod ynamic conditions. Here the morphogenesis during the younger quaternar y was determined by arid conditions and a lowering of the base level: The parts of the pediments consisting of sandstone appeared as broad d ivides, and their debris cover was gradually removed. The parts of the pediments consisting of shale were intensively eroded, so that today only narrow ridges are preserved. Only next to the Fremont River young er rock pediments consisting of shale were formed. They correspond to fluvial terraces 45 m and 35 m respectively 15 m and 12 m above the va lley bottom of the Fremont River; they are dated as Wisconsin respecti vely Holocene. Closer to the mountain front sheet erosion was replaced by linear erosion: the pediments were dissected, some riverlets reach back to the fans and periglacial debris.