Evaluation of Published Criteria for Identifying Metamorphic Rocks on Air Photos: Two Case Studies in the Northeastern United States,
ARMY ENGINEER TOPOGRAPHIC LABS FORT BELVOIR VA
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Published criteria for identifying metamorphic rocks by air photo patterns were systematically tested on regionally metamorphosed rocks in west-central Vermont and near West Point, New York, and were found inadequate for use a predictive tools. Mapping units were discriminated on the air photos by evaluating the pattern elements of landform, drainage-plan, drainage-cross section, photo tone, and photo texture. They were identified by comparing each pattern element to its counterpart for each rock type in the published criteria. The most common pattern element rock name was selected as the name for the mapping unit. To avoid bias, geological data were not obtained until after the identification process was completed. The predicted rocks names were then compared to geologic maps and field data to determine their accuracy. Only 20 of the 74 verifiable predictions were correct. Probable causes for these results are 1 that the published criteria are incomplete and internally inconsistent and do not include all the rocks types found in the study areas 2 that climatic and structural, or tectonic, factors, apparently not considered in the published criteria, play a significant role in determining rock type photo pattern and 3 that the glacial history of these areas has affected the characteristic photo patterns of the rocks encountered. Moderate success was achieved, however, where lithologic diversity and morphic rocks of higher grade andor sedimentary origin. Similar techniques have proven successful for identifying igneous and sedimentary rocks on air photos, so development of more accurate criteria for metamorphic rocks, taking these factors into consideration, is continuing.
- Cartography and Aerial Photography
- Geology, Geochemistry and Mineralogy