i 
JAGGAR: EROSION AND SEDIMENTATION. 291 
of two cascades and a perspective view of the canyons. On the left 
is shown the upper margin of the model and the back slope. The 
three light bands in the cross-section consist of about twenty thin 
layers each of marble dust, coal, ete.; the darker bands are sand. 
During the final days of spraying the model was tilted 10 degrees. 
The important part played by volume of water, as contrasted with 
slope, in producing erosion, is well illustrated in this model. The 
back-slope, shown on the left in Plate 3, had a grade of some 45 degrees, 
while the surface of the model sloped only 10 degrees. All the stream 
development was on the main surface, however, because nearly all the 
water which fell on the model flowed down that surface. Moreover, 
the underground structure sloped the same way, producing seepage 
in that diréction, and away from the backslope. The high divide at 
the back margin of the model remained practically uneroded, and 
what water trickled down the backslope never acquired volume or 
load enough to do any considerable trenching. The backslope would 
be the equivalent of “obsequent” or infacing slopes in a coastal 
plain escarpment. Such slopes are commonly supposed, by reason 
of their steepness, to cause or give evidence of a rapid retreat of the 
escarpment. No sign of such retreat was observed in this model, 
and all of the hydrostatic conditions indicate that such a slope would 
absorb moisture and carry the water down the dip of the strata. It 
would yield no springs for the development of “obsequent” streams. 
It is of interest to note in the structure of the sloping frontal delta 
beds the following sequence :— 
Thick lower white marble dust layer. 
Sand layer. 
Second marble dust layer. 
Upper sandy layers. 
These correspond to the materials of the model in the order in which 
they were reached by the eroding streams, viz:— 
Thick upper marble dust layers. 
Sand layer. 
Second marble dust layers. 
Sand layer. 
The lowest group of light colored layers in the original model forms 
the bottom of the deepest canyon sectioned. Its effect is shown in 
the lighter color of the highest delta beds seen farthest to the right. 
On a steep submarine continental slope receiving the detritus of many 
rivers, some such corresponding inverse sequence of beds eroded and 
beds deposited might be looked for. 
