Ch. 13] PERMEABILITY 237 



exposed to tidal scour have been described (Koppejan, Van Wamelen, 

 and Weinberg, 1948). 



Gross Structure 



Clay minerology and the structure of fine-grained materials play a 

 prominent role in the stability of poorly consolidated materials, but 

 the previous discussion may have overemphasized these particular 

 points because of the interesting advances in current research. 



Homogeneous deposits of clay or silt are uncommon, and it is gen- 

 erally necessary to take into account many other factors that bear 

 on the arrangement and grosser structures of the deposit before a 

 rational approach to analysis and cure of a slide can be made. In 

 particular, the stratigraphic sequence, the attitude of the strata, and 

 the presence of discontinuities such as joints, faults, and bedding 

 planes exercise their own kind of control. The influence of these large- 

 scale structures is generally apparent after short study of any par- 

 ticular slide. Their action is well-known and has been often described; 

 hence it will not be greatly detailed below. 



Flat-lying stratigraphic series in which massive beds are interstrati- 

 fied with or underlain by shales or clays and exposed by erosion into 

 steep slopes are commonly subject to landslides if the shales are wet- 

 ted. Climate and the stratigraphic sequence of alternating massive 

 sandstones and limestones with shales have produced erosion forms 

 particularly susceptible to landslides in the Colorado Plateau (Hinds, 

 1938; Reiche, 1937; Strahler, 1940; Varnes, 1949). Massive volcanics 

 interbedded with or overlying shales have been involved in many slides 

 in the canyons of Idaho (Russell, 1901) and in the San Juan Moun- 

 tains of Colorado (Cross, 1899; Cross and Spencer, 1900; Howe, 1909). 



Permeability 



Permeable beds or lenses within fine-grained deposits are especially 

 dangerous, for they allow water to reach and lubricate large surfaces. 

 A slide in interbedded shale and sandstone at Soldier Summit, Utah, 

 in a cut of the Denver and Rio Grande Western Railroad was prob- 

 ably aided by water from an underlying porous sandstone. Owing to 

 dip and impermeable cover, the water in the sandstone bed was under 

 considerable pressure at the site of the slide. If the permeable beds 

 are also loose, the danger of internal erosion and loss of support is 

 added to the possibility of failure by rotational shear slip. An inter- 

 esting example of the influence of a permeable sand bed on a landslip 

 on the south coast of England is described by Ward (1945, 1948). 



