THE GEOMETRY, DESCRIPTION AND PROPERTIES OF ROCK MASSES

Rock structure is a term used to describe the overall relationship of rock masses, for example folding, jointing, cleavage, and unconformities. Folding of rock changes the orientation of rocks by lateral compression, the changes perhaps being most apparent in bedded rocks. The compressive forces causing folding, and indeed fracture structures as well, originate most usually at destructive plate margins. The tilting of rocks from their original horizontal position, and the fracturing of rocks, produces measurable directional properties in the rocks called strike and dip (Fig. 3.1). The strike is the direction at any point on a structural surface of a horizontal line drawn on the surface. The term is also used in the sense of the general trend of rocks. It is at right angles to the true dip. True dip is the vertical angle measured from the horizontal plane in the direction of greatest slope. The apparent dip is the dip in any direction from zero in the direction of strike to a maximum in the direction of true dip. The measurement of these properties by geologists is in terms of strike, measured as a whole compass bearing from north (for example, a bed striking NE-SW would have a strike of 045° or 225° — they are exactly the same), a dip angle, which is self explanatory, and a general dip direction, usually as one of the eight basic cardinal points of the compass. So, in Fig. 3.1, if north is in a horizontal plane passing directly into the page the bed illustrated would have the following approximate orientation…