Tree-ring dating in several avalanche tracks in Galena Creek valley, northern Absaroka Mountains, Wyoming, is used to determine the frequency of large snow avalanches that pass below the forest line. The following criteria are used: (1) datable scars on the trees, (2) changes in growth-ring pattern from concentric to eccentric, caused by tilting, (3) changes in growth rate due to increase in photosynthesis when adjacent trees are destroyed, and (4) age of trees within a given reforested avalanche track. The first two of these criteria are most reliable. Many young trees within the avalanche tracks are protected by snow during avalanches, and they thus survive to reforest the track immediately following destruction of the larger trees. Above the forest line, avalanche boulder tongues are one of the most reliable indicators of persistent activity in alpine regions, for they are formed over a period of many years by the accumulation of debris swept out of avalanche chutes. A peculiar linear feature on the surface of the tongues is the avalanche debris tail, which consists of fine debris that was deposited by large snow avalanches downslope 5 to 10 m from a large boulder. They are thought to be formed by a mechanism similar to that by which sand shadows are formed in the lee of obstacles in river channels or on desert dunes.

Abstract The lower Paleozoic conglomerates of the Southern Uplands, deposited in areas of rapid subsidence , are commonly coarse grained and of varied thickness. They contain a great variety of clasts, including mainly Arenigian volcanic rocks, other igneous rocks not now found locally, and metamorphic rocks of Dalradian aspect. The source area for the conglomerate appears to have been a roughly NE-SW-trending ridge called “ Cockburnland ,” situated north of the geosyncline. North of Cockburnland , two conglomerate bodies of Silurian age , derived generally from the south, also contain clasts of igneous rocks (Arenigia n type) and of Dalradian -type metamorphic rocks. The Lower Old Red Sandstone conglomerate beds exposed along the southern margins of the Midland Valley contain fragments of lower Paleozoic graywacke derived from the uplifted geosynclinal strata farther south; conglomerates of the northern margin contain High land rocks derived from the north. More detailed studies of the Upper Old Red Sand stone in the vicinity of the Firth of Clyde show that the conglomerates there be long to several dispersal systems, three of which are exposed along the coast between Ardrossan and Gourock. Each dispersal unit (shadow ) comprises an evolutionary environmental sequence from alluvial fans to braided streams and , finally, to floodplains of meandering streams. There is thus an extension of the source area and the depositional area with time. The southernmost dispersal shadow contains sand stone clasts within the conglomerate; the northernmost contains schist. The interm ediate shadow is typified by volcanic fragments.