OCR Text |
Show - 4- hoar layer. This cause of instability was recognized at the time of the Figure 7 slide, so that extensive control measures were undertaken following subsequent snowfalls in anticipation of more instability. This anticipation was fully justified on 24- 25 January, when extensive avalanching occurred throughout the Wasatch Mountains following a 130- cm snowfall. The entire snow cover of Figure 7 is so weak that the ram profile offers no stratigraphic information. Figure 8 is an uncomplicated case of wind slab resting on fragile depth hoar. Sliding surface at the fracture line was the upper part of the depth hoar coinciding with tops ; of the rocks scattered across the slope. Once this slide picked up momentum, it removed all the snow cover back to the ground. Here the unstable configuration of a slab layer on depth hoar can be discerned from the ram profile. In summary, the following is concluded from this brief study of fracture line profiles from the Middle Alpine zone: 1. Structural weaknesses in the snow cover leading to climax avalanching can readily be recognized by visual inspection of a pit wall. 2.. The ram penetrometer alone is not a satisfactory instrument'for detecting these weaknesses under the snow and climate conditions found at Alta. The snow is often too soft or weak to reveal much stratigraphy to the penetrometer, and when it does, the clues to instability may be obscure. ( Compare this in contrast to the Berthoud Pass records, for instance Figures 32- 35 in " Snow Avalanches."). 3. Avalanche release often originates with a thin layer of very low shear strength- the lubricating layer. This observation is based on general experience as well as the present collection of fracture line profiles. |