Geology Reports⌕ Search

Geology topics

Robert K. Fahnestock

Publications and source records attributed to Robert K. Fahnestock.

3 recordsLinked to original sources
USGS1965

Rockfalls and Avalanches from Little Tahoma Peak on Mount Rainier, Washington

In December 1963 rockfalls from Little Tahoma Peak on the east side of Mount Rainier volcano fell onto Emmons Glacier and formed avalanches of rock debris that traveled about 4 miles down the glacier and the White River valley. In this distance, the rock debris descended as much as 6,200 feet in altitude. Minor lithologic differences and crosscutting relations indicate that the rockfalls caused at least seven separate avalanches, having an estimated total volume of 14 million cubic yards. The initial rockfall may have been caused by a small steam explosion near the base of Little Tahoma Peak. During movement, some of the avalanches were deflected from one side of the valley to the other. Calculations based on the height to which the avalanches rose on the valley walls suggest that their velocity reached at least 80 or 90 miles per hour. The unusually long distance some of the avalanches were transported is attributed to a cushion of trapped and compressed air at their base, which buoyed them up amid reduced friction.

Bulletin↗
USGS1962

Flume studies of the transport of pebbles and cobbles on a sand bed

During experiments on sediment transport and resistance to flow with a uniform 0.33-mm sand , data were recorded on the movement of individual rocks having intermediate diameters from about 0.1 to 0.5 foot. The experiments were conducted in a flume 2 feet wide by 60 feet long and for most runs, depth was held constant at 0.5 foot. The experiments showed that rocks on the sand bed moved downstream consistently only if the flow was in the upper regime - that is, only if the bed forms were plane bed , standing waves, or antidunes. The rocks moved at velocities that were approximately half the average velocity of the water. On all bed forms in the lower flow regime (ripples, ripples superimposed upon dunes, and dunes), the rocks moved upstream and down into the bed . That is, the rocks moved into a scour pocket that formed at the upstream side of the rock. The movement upstream and down into the bed is limited by and approximately equal to the distance below the original rock position of the minimum bed elevation plus approximately half the rock diameter. The data indicate that cross-bedded sand deposits formed by the ripple or dune phases of transport would contain few, if any, pebbles or cobbles . Because the flow, in at least the downstream reaches, of most rivers is in the lower regime, the upstream movement and scour into the bed demonstrated in these experiments is an important factor in the sorting process.

Geological Society of America Bulletin↗