Coarse Clastic Rocks 



(Plotted in canary yellow) 



This group of rocks consists of gravels, conglomerates, sands, and sand- 

 stones. Grain sizes range downward to 0.1 millimeter, and the grains may be 

 of any noncarbonate material. In describing the coarse clastic rocks, one should 

 consider the following characteristics: 



1. Grain Size: Grain sizes should be determined to the nearest 0.1 milli- 

 meter. Record the weighted average size. In cases where the largest grains are 

 much larger than the weighted average, record also the maximum size of the 

 grains. 



2. Grain Shape: There is an almost infinite number of variations in the 

 shapes of sand grains, but for the sake of simplicity and consistent recognition, 

 only 5 general classes are considered. These typical shapes are shown, in Figure 

 3-3 and may be defined as follows: 



SHARP: Conchoidal surfaces terminating in sharp edges and corners. 

 ANGULAR: Flat, plane surfaces, generally terminating in acute or right angles. 



Edges commonly thin to sharp. 

 SUBANGULAR: Flat plane surfaces terminating in well-rounded edges. 

 ROUNDED: Generally rounded surfaces, broadly rounded edges and corners. 

 GLOBULAR: All surfaces convex. Nearly equidimensional. Spheroidal. 



The roundness of a grain has little to do with its over-all dimensions. A 

 perfectly rounded grain can be elongated or flattened if rounding has progressed 



Figure 3-2. A — Plan for cutting sheet-metal scoop tray for viewing samples under the 

 microscope. The material, preferably aluminum, can be cut with tinners' shears. B — 

 Completed tray with 0.5 millimeter grid for measuring sand grains. 



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