Identifying Landforms and Soils by Aerial Photographs

In the process of analyzing aerial photographs to obtain information on the engineering characteristics of soils, the first and perhaps the most important step is to identify the landform. The landform may consist of hills, valleys, plains, terraces, beaches, dunes, and other topo­ graphic features largely created by the forces of erosion. The im­ portance of the landform lies in its relation to the soil that has been produced by the weathering action that has played a part in shaping the features of the terrain. Within a given area there usually will be found two types of landform : destructional and constructional. The hills of high ground are being worn down by erosion, while the flood plains of streams are built up by deposition of the material eroded from the upland. For the best results from photo-interpretation, particularly in areas where the inter­ preter’s experience is limited, sufficient photo-coverage should be avail­ able to include both constructional and destructional features. This is especially true where the survey is expected to include the location of gravel and sand deposits, since they are most often found within areas of constructional landforms. The landform is the key to the type of parent material, and the parent is the key to the general physical properties of the soil. There­ fore, it is important first to identify correctly the landform. In doing this, the angle of repose of weathering slopes is highly significant. W ith­ out going into details, most observers w ill credit the more resistant materials with an ability to maintain steep slopes, while the less weatherresistant materials assume “soft” slopes. Consequently, when an area of very gentle slopes is examined, we can tentatively assume an easily