A FRACTURE SYSTEM MODEL 171 



"While all orographic bloclis have iu some degree been either elevated or 

 depressed with reference to all of their neighbors of the same order, yet, since 

 the amount of the vertical displacement is different along different fault planes 

 of the same series, with larger throws at regular intervals, it follows that 

 blocks will group themselves into composite blocks of a higher order. Such 

 composite blocks will generally be outlined upon the map, either geologically 

 by the boundaries of exposures of a given formation or in the elevated bound- 

 aries of a very resistant formation, or topographically by sudden changes in 

 the profiles of physiographic features. These composite blocks may be of 

 several orders, some of the larger in the Pomeraug Valley being indicated in 

 the serrated eastern boundary." (See figure 29, page 156, and figure 38, page 



A FRACTURE 8TSTEM MODEL 



To properly represent such an elaborately faulted system as that of the 

 Pomperaug Valley in the form of a model would be a more or less difficult 

 matter in view of the considerable 



nimiber of fault series there dis- , \\ V''' 



pla3^ed. On the other hand, the ele- x';\-<^(^\ 



ments in the primary pattern which ; --^^^/f^--^ 



have been above described would in- / , i ^-^^^^^'^ 



volve few difficulties, while bringing / vv^^P^\ ^° is-° 



out the relation of composite to unit -.^ \L^^ '' \ \ / j-f 



blocks and the nature of displace- \ "^(^ ' \ l y^ 



ments. Simplifying this pattern ^^-^^^y.^ ' ^^' 



still further by the omission of one '' ^ \ ' e5--R.oa.. 



set of fractures, as quite often occurs figure 41.— Details of Faulting along 



in nature, its more important char- ^^;o^* ^f Basalt cuff shoivn at a a in 

 acteristics may be displayed by a 



, . , . -^ ^ T T / 1 (Hobbs, U. S. Geological Survey) 



relatively simple model (see plate 



13, figure 2). In this model the smallest identifiable fault compart- 

 ment — the "unit" block — is represented by the smallest unit of the modeL 

 Composite blocks of the next higher order, the second, are comprised of 

 four rows of three blocks each. The blocks of the second order, it will 

 be observed, group themselves into three rows of two each for the third 

 order blocks, and these in turn into larger blocks containing similarly 

 three rows of two each. Such a composite structure, if it controls erosion, 

 must develop repeating patterns similar in character to those above de- 

 scribed from so many widely separated districts. 



SUDDEN CHANGES OF THROW ON FAULTS 



If the above described model correctly represents the nature of a pat- 

 terned fault system, which is itself made up from and is a pait of the 



62 Twenty-first Annual Report of the U. S. Geological Survey, loc. cit., pp. 110-111. 

 XIII— Bull. Geol. Soc. Am., Vol. 22, 1910 



