Structural geology . The dark portion on the left is a bed of softer shaly quartzite. The outcrop isa part of the north limb of a syncline. The right hand bed is on the south. It hasobviously moved upward with reference to the beds to the north of it, as would beexpected from this position on the syncline. The fractures here have been de-veloped by rotational or .shearing stresses described on pp. 16, 20. It is suggestedthat the student superpose on these beds the theoretical positions of the strain el-lipsoids and the planes of maximum shear. Note relations of fracture cleavage tojointing in
Structural geology . The dark portion on the left is a bed of softer shaly quartzite. The outcrop isa part of the north limb of a syncline. The right hand bed is on the south. It hasobviously moved upward with reference to the beds to the north of it, as would beexpected from this position on the syncline. The fractures here have been de-veloped by rotational or .shearing stresses described on pp. 16, 20. It is suggestedthat the student superpose on these beds the theoretical positions of the strain el-lipsoids and the planes of maximum shear. Note relations of fracture cleavage tojointing in adjacent bed. (See also page l-lj. 24 COMPRESSION JOINTS 25 formed by compressional shearing, caused by slipping between thebeds. Short open gashes or joints are also developed here by ten-sion, as indicated in the figure. (d) The sheet structure so commonly observed and utilized ingranite and other quarries is a system of jointing probably at leastin part developed under compressive stresses. (Figs. 12, 13 and 14.). Fig. 10. Fracture cleavage developed in slaty quartzite layer between two massivebeds of quartzite, on south limb of the Baraboo syneline, Wisconsin. Note thedirection of differential movement and correlate this with position on the are the relations of the cleavage to pressure? Note relations of fract-ure cleavage to joints in the adjacent massive layers. (See also Fig. 37 andpage 121). The sheets are thinnest near the surface and rapidly thicken may be curved, and in general are parallel with the rock sur-face. Usually they are found to be lens-shaped when traced somedistance. Many instances have been noted of a lengthening ofblocks when quarried out, sometimes with explosive violence,indicating that in the ledge they were under compressive stress. 26 STRUCTURAL GEOLOGY Compression is indicated also by the occasional flattening, by-faulting, of drill holes and other The sheet structure isdeveloped artificially by the use of explosi
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