158 Subsurface Geologic Methods 



Field Work and Its Presentation 



Some of the more important papers on petrofabric methods and pre- 

 senting information on petrofabrics gathered in the field, are the follow- 

 ing: "The Application of Recent Structural Methods in the Interpretation 

 of the Crystalline Rocks of Maryland," by Cloos,^^ a volume that contains 

 not only Cloos' paper on methods but also a number of supporting papers 

 by his students; "Lineation," by Cloos,^^ dealing exclusively with linea- 

 tion, its formation, interpretation, and mapping; "Oolite Deformation in 

 the South Mountain Fold, Maryland," also by Cloos,^^ which is a detailed 

 account of the use of deformed ooids in interpreting the structure of an 

 area; and "Structural Petrology of Deformed Rocks," by Fairbairn,^^ 

 which is a detailed account of petrofabrics, its interpretation, and presenta- 

 tion, chapters 1 through 7 dealing with the theoretical aspects of the sub- 

 ject. 



The presentation of petrofabric features is accomplished through the 

 use of symbols. The United States Geological Survey has recently pub- 

 lished a "New List of Map Symbols" that may be obtained free of charge 

 from the Geological Map Editor, United States Geological Survey, Wash- 

 ington 25, D. C. These symbols were submitted by a committee composed 

 of Ernst Cloos, L. B. Pusey, W. W. Rubey, and E. N. Goddard, Chairman. 

 Reproduced on pages 160 to 163 are the symbols from this list that are 

 most frequently used in mapping petrofabric data. 



Plastic flow in crystalline material takes place by intergranular lattice 

 displacement and rotation and migration of materials without destroying 

 the cohesion between the grains of the rock subjected to deformation. 

 Rocks thus deformed are called "tectonites," whereas all others are classed 

 as "nontectonites." 



In order to carry out the correlation between the field data and micro- 

 scopic data, the structures of the rock are referred to three coordinates 

 or axes a, b, and c, each normal to the other. Figure 60 illustrates a plung- 

 ing anticline with its elements tied to these axes. From this diagram it is 

 seen that a is the direction of movement or transport, b is the fold axis, 

 and c is the vertical component. The a-c plane is normal to the b direc- 

 tion. 



While field mapping is in progress, oriented specimens are collected 

 from the area for later microscopic study. These are so marked (fig. 61) 

 that, once in the laboratory, they are reoriented to their proper position 

 in space; thus, the microdata may be correlated with the a, b, and c 

 structure axes. 



Figures 62 to 67 show tight folds in bedded, schistose quartzite of the 



^- Cloos, Ernst, The Application of Recent Structural Methods in the Interpretation of the Crystalline 

 Rocks of Maryland: Maryland Geol. Survey, vol. 13, 295 pp., 1937. 



°" Cloos, Ernst, Lineation: Geol. Soc. America Mem. 18, 1946. 



^* Cloos, Ernst, Oolite Deformation in the South Mountain Fold, Maryland: Geol. Soc. America Bull., 

 vol. 58, pp. 843-918, 1947. 



^ Fairbaim, H. W., Structural Petrology of Deformed Rocks, Cambridge, Mass., Addison-Wesl^y Press, 

 Inc., 1912. 



