Rock Fractures Called Joints 



by A. W. Bcaslev* 



I'xposures tjf rocks on the surface 

 of our Earth generally owe their form 

 to the local joint pattern. A joint is a 

 break in a rock mass where there has 

 been no relative movement of rock 

 on opposite sides of the hreak. Many 

 ocks are Intersected by regular 

 parallel sets of joints which arc 

 imi.iIIs closely spaced. Where several 

 HttS Of joints are developed, the rocks 

 lend to hreak into cuhic or rec- 

 tangular-shaped blocks the size ol 

 which reflects the spacing ol joints in 

 ihe sets; it can range from a few 

 inches lo several yards. The term 

 joint is said to have originated among 

 British coal miners who were re- 

 minded of mortar joints that bind 

 bricks or stones together in a vs all. 



Rocks split along joints and 

 It being very important 



velopment of our landscape, they are 

 important in many of man's activities. 

 Rock excavations, tunnels, and quarry- 

 ing operations must lake account of 

 the spacing of the joints. Small blocks 

 can be quarried nearly everywhere, 

 but the number of possible sites for 

 quarrying blocks of building stone 

 more than a yard or so in dimension 

 is small. Joints are also useful to the 

 geologist (both professional and 

 amateur) in the field in providing 

 natural smooth surfaces on exposures, 

 generally in several directions, on 

 which the internal structures ol the 

 rocks can be observed. In sonic par- 

 ticularly massive rocks, such as 

 granites, hand specimens would be 

 difficult to collect were joints absent- 

 Water flowing over a rock BtVfacfe 

 lends 10 travel along the joint partings. 



S;,,Ml-.lnnei.aversc.-1 by joints 

 December, 1971 



K Neck; Tasmania. 



mva: R. F. Cwic 



347 



