SEDIMENTS OF THE AVON AND OTHER RIVER8 327 



a portion of an area in which very soft heds were interpolated between 

 more coherent strata, the more mobile beds would be likely to squeeze 

 outward away from the sand pressure toward an unsupported edge, if one 

 were developed by stream or wave cutting. This might occur without 

 distui'bing firmer beds aliovc and below tbi-ongh tlie more yielding cliar- 

 acter of the soft beds. 



Figure 3, whicli is based on a section of t1ie deposits of the iMissoiiri 

 River at Blair, Nebraska, shows an arraugcmeut of mud and saud l)eds 

 which will aid in understanding the uiechanical conditions under which 

 these soft muds miglit be sealed between beds of sand which might hohl 

 for an indefinite period the potential power of deforming t1iem. A small 

 amount of bottom cutting by the river would expose the mud-beds on the 

 right side of the section and cause them to be squeezed lateral 1\' into the 

 river by the heavy sand load above them. Such horizontal movement of 

 the material of the soft mud-beds would inevitably produce deformation 

 of tlie lamina^ consequent on fiowage of the beds behind them. 



Artiftctal Deformation of soft Beds • 



The behavior of soft beds overlain by une(]ually distributed firmer and 

 heavier beds, as observed in an experimental tank, is instructive in this 

 connection. Marked deformation of the soft beds was thus induced by 

 differential weighting of the firmer upper beds in a small tank. Two 

 beds of clay, each about three-fourths of an inch thick, were de[)osiled 

 over the bottom of the tank by introducing the clay into the partially 

 water-filled tank by means of a tube connected with another vessel used 

 as a reservoir for the thoroughly mixed clay and water. Two thin layers 

 composed respectively of powdered charcoal and plaster of paris, which 

 was much more resistant than the clay beds, were laid on top of the first 

 bed of clay. These show in figure 4 as white and black bands in the 

 lower half of the section. On the level surface of the upper clay bed a 

 delta was formed at one end of the tank by running into it a small stream 

 of sand-bearing water. This delta, as it was formed, sank into and nearly 

 through the upper clay bed, pressing it laterally and considerably thicken- 

 ing it near the sides of the delta. A second delta Avas made of fine sand 

 at the opposite end of the tank. The bottom-set beds of this delta spread 

 over the entire surface of the clay and of the other sand delta, which had 

 covered but a small part of the clay. The two fan-shaped deltas thus 

 developed covered the middle and sides of the clay bed with a layer of 

 sand one-eighth to one-half of an inch thick, while a small area of the 

 clay at each end of the tank supported a sand deposit one to two inches 



