72 



PETROLOGY AND PEDOLOGY: 



origin, and whether or not the individual particles 

 are cemented together. On the basis of origin these 

 rocks are termed clastic rocks if they are the products of 

 weathering. If the individual particles are over 10 

 inches in diameter, the rocks are called boulders; if be- 

 tween 2^2 and 10 inches, cobbles; if between 1/6 and 

 2}/2, pebbles; if 1/6 to 1/25 inch, granules: if 1/500 to 

 1/25 inch, sand; if 1/5000 to 1/500 inch, silt: and if 

 smaller, clay. However, cobbles, pebbles, and granules 

 are usually grouped as gravel, and silt and clay as 

 clay. 



Clastic rocks are also differentiated on the basis 

 of their primary agent of erosion. Unassorted 

 angular, polished or scratched boulders, pebbles, and 

 finer particles left by glaciers are called till. Coarse 

 fragmentary rocks produced by weathering are called 

 talus. Coarse rocks rounded by water are called 

 gravel. Finer rocks, rounded and deposited by water 

 or wind are called sand. The finest materials, if de- 

 posited by water, are called clay; if deposited by wind, 

 they are called loess. Further types of clastic rocks are 

 formed when the above types are cemented. Ce- 

 mented till is tillite: cemented gravel is conglomerate: 

 cemented talus is breccia; and cemented clay or loess is 

 shale. 



The other sedimentary rocks, according to origin, 

 are called nonclastic. They are formed by decomposi- 

 tion, dissolution, and redeposition of other rocks and 

 organic matter. Limestone is composed mostly of 

 calcium carbonate remains of plants and animals. 

 Small, soft and porous; cemented particles are called 

 chalk and cemented combinations of chalk and clay 

 are called marl. Also, the spongy, porous, or earthy 

 limestones accumulated at the mouths of springs are 

 called tufa, and the solid, often layered, limestone 

 deposited in caves is called travertine. The different 

 travertine layers may or may not be of different 

 colors. 



Other nonclastic rocks are common. Deposited 

 silica is called chert. The mass of siliceous skeletons 

 of diatoms (microscopic algae) is called diatomite, 

 or diatomaceous earth. Carbon from plants, by proc- 

 esses of decomposition and progressive compaction, 

 occurs as peat, lignite, or coal. Phosphate from ani- 

 mals or igneous rocks gathers as phosphate rock; 

 however, phosphate often is deposited with carbon- 

 ates and is incorporated within limestone. Com- 

 pounds, mainly from animals, provide petroleum and 

 asphalt. Various salts are also collected, the most 

 common being gypsum and halite, a hydrous calcium 



sulfate and common table salt. The accumulation of 

 excrement and skeletal remains of certain animals, es- 

 pecially birds and bats, is guano. Finally, iron in 

 solution may dehydrate to form yellow ochre, or 

 limonite, and red ochre, or hematite. 



SEDIMENT STRUCTURE 



Sediments usually are layered into beds that are 

 separable from one another. In other words, each 

 stratum normally possesses unique structure. The 

 exact character of individual layers is dependent 

 upon many environmental factors (e.g., current varia- 

 tions, seasonal and long range climatic changes, sea 

 level fluctuations, and organism number and type 

 changes). Due to these factors, stratification can be 

 (1) cross-bedded, sediments laid down in one or more 

 angles to the horizontal; (2) graded, consist of strata 

 of mud, sand, and gravel that grade into one another; 

 or (3) lens-shaped. 



In addition, sediments and therefore layers can 

 show the influence of other environmental features. 

 For example, mud cracks occur where mud shrank 

 and dried; ripple marks where waves traveled over a 

 shallow bottom, or where wind traveled over sand; 

 rill marks where ocean waters returned to the sea, 

 or rain ran downhill; raindrop impressions where 

 rain fell; and wave marks where coasts were lo- 

 cated. Finally, fossils are frequently found in strata. 

 These remains naturally indicate the kinds and num- 

 bers of plants and animals present during; the period 

 of deposition. 



After sedimentation, the deposits may be altered 

 secondarily. The commoner secondary alteration 

 products are concretions, geodes, and stylolites. 

 Concretions are variably shaped, seemingly foreign 

 rock, within a stratum. They probably develop by 

 dissolution of rather uncommon materials from the 

 parent stratum and concentration of these materials 

 about some object. Some concretions are cracked by 

 jointing and then the cracks are filled by other sub- 

 stances. The resulting products are called septana, 

 or septarian concretions. 



6>o^M are cavities filled with crystals. The crystals 

 grow from the edge toward the center of the cavity 

 and probably as a result of chemicals in solution en- 

 tering a cavity. Veins are very firm bindings be- 

 tween partings, or cracks, in any kind of rock, es- 

 pecially limestones. The binding material usually is 



