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SYMPOSIUM PHOTO INTERPRETATION, DELFT 1962 
Parent material refers to the core material within the land form. There are 
two groups of parent materials - the consolidated materials or rock and the 
unconsolidated materials or drift. These are subdivided according to environ 
ment of origin. 
Tables 3, 4, and 5 are tabulations of the consolidated parent material types. 
Table 3 is a broad classification of igneous rocks into plutonic and volcanic 
groups with emphasis on texture and color which control the airphoto pattern. 
Table 4 is a broad classification of sedimentary rocks emphasizing clastic and 
non-clastic origin. Table 5 is a broad classification of the metamorphic rocks 
with emphasis placed on the original rock which the metamorphics so closely 
resemble as does the airphoto pattern. 
The consolidated parent material type can be related to the core material 
of the rock land form type by airphoto interpretation. Once identified, it is 
used to modify the land form type to differentiate the land form - parent material 
type into discrete units. For example, a limestone plain, a shale plain, a basalt 
plain, a rhyolite tongue, a granite shield, a gabbro shield, etc., are all discrete 
land form-parent material types. Basic and supplemental ground control from 
airphoto sample areas is important in this interpretive process. 
The unconsolidated parent material classification is presented in Table 6. 
It pertains to the drift types. The question ofsematics may be resolved without 
change in the classification by substituting the term, deposit or sediment, for 
the term, drift. 
This particular classification is the author’s, but it is based on a modification 
of the sedimentary environment presented by G. P. Merrell in his 1906 
“Treatise of Rocks, Rock-Weathering and Soils”. The engineer is interested in 
this earthen material before diagenesis; therefore, its unconsolidated classi 
fication is of great importance. 
The drift parent material types are invariably combinations, in various 
proportions, of individual particles - such as gravel, sand, silt, and clay. In the 
author’s experience, a silty alluvial drift, a silty eolian drift, a silty lacustrine 
drift, or a silty glacial drift may perform differently and require different 
engineering designs. They may have different densities, strengths, arrange 
ment of particles, shape of particles, physical-chemical properties, etc., that 
warrant their separation. A classification of this type is a warning to engineers 
that special explorations and studies are required before designs are developed. 
The airphoto patterns of consolidated and unconsolidated parent materials 
involve the entire subject of pattern elements. All variations in topography, 
drainage plan, erosional shape, tones and textures related to land use, tones 
related to vegetation and tones related to materials are important. 
Classification of engineering soil groups 
The land form type and parent material type do not completely differentiate 
an area into discrete units. Within any land form-parent material unit there 
will be distinct variations in texture of material produced both by origin,