Observations of sand transport and rapid deposition by strong sand-lad
en surges in an open channel, by means of highspeed motion pictures, r
evealed that the dominant effect is a process by which a laminar shear
ed layer, with sand concentrations approaching the threshold for immob
ilization by grain interlocking, develops as suspended sand becomes co
ncentrated near the base of the flow. The laminar sheared layer climbs
vertically with time as sediment is progressively immobilized at the
base of the layer and added at the top from the overlying turbulent fl
ow The mobility of the laminar sheared layer is probably enhanced by c
ontinuous upward flow of interstitial water that is trapped within the
layer as the layer accumulates. As the flow weakens and the concentra
tion of suspended sand in the flow decreases, the laminar sheared laye
r thins and/or becomes immobilized, giving way to the familiar weak tr
action transport on a well-defined immobile sand bed. Particles reside
in the laminar sheared layer only briefly, and move only a short dist
ance before immobilization; the laminar sheared layers seem not to sha
re essential features with traction carpets. Much of the coarser, stru
ctureless lowermost parts of thick turbidites might have been emplaced
by processes not unlike those described here. Our results suggest tha
t the entire turbidity current need not be of extremely high density f
or an initial deposit to be formed rapidly from extremely high near-be
d sediment concentrations.