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- 1311 - 
Analyzing the regional geology is an effective way to initiate develop- 
ment of the three-dimensional concept of the terrain. Regional land- 
scapes have been divided into physiographic provinces; that is, distinct 
regions which have undergone similar geologic history and are composed 
of similar terrain features. These physiographic provinces can then be 
rated with respect to their incidence of landslide occurrence, suscep- 
tibility to landslides based on historical records, and experience. A 
recent rating of physiographic provinces of the United States, with 
respect to landslide severity, was developed by Radbruch-Hall and others 
(9). Their map depicts areas of high, moderate and low landslide 
incidence, and areas of high and moderate landslide susceptibility. 
The accompanying text discusses the slope stability characteristics of 
the various physiographic Provinces, and the geologic formations and 
geologic conditions that favor landsliding in the various provinces. 
The analysis of the regional geology and delineation of the physiographic 
provinces are best accomplished with small-scale aerial photography and 
imagery. Great success has been achieved at this stage with high- 
altitude color infrared photography, and satellite and radar imagery. 
The information developed at this stage, however, is fairly general. 
A more detailed investigation is required to pinpoint the actual slides 
or vulnerable locations. 
In evaluating physiographic provinces in more detail, the natural land- 
scape is divided into distinct units called landforms. The designation 
of a specific landform connotes both a genetic classification and a type 
of landscape. For example, a sand dune landform denotes deposits formed 
by wind movement and sorting, which form unconsolidated, smooth, flowing 
hills and ridges. An appreciation of the genetic aspects of landforms 
enables one to estimate their potential susceptibility for movement. The 
type of landscape developed by each landform provides a basis for separating 
the various landforms and thus recognizing those most prone to sliding. 
Landslides can occur in almost any landform if the conditions are right 
(e.g., steep slopes, high moisture level, no vegetative cover).  Converse- 
ly, landslides may not occur on the most landslide-susceptible terrain 
if certain conditions are not present (e.g., clay shales on flat slopes 
with low moisture levels). Experience in observing and working with 
various landforms, however, has demonstrated that landslides are common 
in some landforms and rare in others. Table 1 provides a key to land- 
forms and their susceptibility to landslides. The subdivisions are 
based on topographic expression and, in the case of hilly terrains, also 
on drainage patterns. This table gives only those landforms in which 
landslides are most common and is not meant to be all inclusive. Almost 
all landforms rated as highly susceptible to landslides are composed of 
alternate layers of pervious and impervious materials (rock or soil), a 
fact that needs to be specifically recognized. The identification of 
landforms are best accomplished by the analysis of medium-scale aerial 
photography (1:20,000 to 1:40,000).