TECTONICS/Fractures (Including Joints) 359 fracture often ends abruptly against another This abutting relationship gives the relative age of the fractures, i.e., the later fracture abuts against the earlier fracture If an early fracture is an open fracture then it will represent a free surface within the rock and, as discussed in the above section on classification of faults, will be unable to support a shear stress Consequently, the principal stresses will reorient as they approach it into a position either normal or parallel to the fracture This effect can be clearly seen in Figure 14, where later fractures curve into an orientation at right angles to the earlier fracture as they approach it Fracture Networks Figure 13 A fracture network in a Liassic limestone bed from Lilstock, North Somerset, England It was produced by the super position of individual fracture sets Structural geologists study the cross-cutting relationships of different fracture sets in order to determine their relative age A variety of rules have been established to help in this task It is found that early fractures tend to be long and relatively continuous and, as noted above, later fractures abut against these and are consequently shorter Some of these features can be seen in Figure 15, which shows a fractured limestone pavement containing several fracture sets The Figure 14 Details of the limestone pavement shown in Figure 13 illustrating the interaction of late fractures with early frac tures The effect of early fractures on later ones is to arrest their propagation and to modify their orientation It can be seen that the later fractures are deflected by and abut against the earlier fractures Figure 15 Fracture patterns in a limestone pavement at Lil stock, North Somerset, SW England The older fracture sets are the most continuous and, as the sets become progressively younger, they become less continuous and less well oriented