roof falls could be effectively controlled by the simple process of adopting and following a systematic method of support. This is seldom the case, in fact a rarity, as mining personnel is well aware, and the roof will usually contain irregularities as shown in Figure 2. As illustrated in these sketches, the roof and even the individual coalbed can be composed of irregularities such as kettle bottoms, cutouts, horsebacks, rolls, partings, and even clay veins which are not uncommon in some areas. The basic types of sedimentary rocks associated with mine roof are sandstone, limestone, and clay. The best known or most common type of roof is a mixture of varying amounts of sediments composing the basic types and are known as shales. Figure 3 is a diagram showing the relationship of the principle sediments of mine roof. These rocks were formed by the accumulations of sand, silt, mud, lime ooze, and partially decomposed organic material. After accumulation, these sediments became covered by other deposits and became compacted by weight of overlying material. Once consolidated, these rocks became more firm due to cementation and compaction, but, often with the passage of time, became weakened due to some process of weathering, usually by the process known as "leaching" or water filtering through the strata. If the rocks lay near the surface, exposure to various agents of erosion, such as frost, wind, water, and air resulted in a weakening of the Weakening of consolidated rocks may develop as the result of earth movements, either of a widespread degree or of a local nature. During the period of consolidation, some of the stronger rocks, such as sandstone, often caused deformation and distortion of weaker rocks, such as shales and coal. Earth pressures of large magnitude often resulted in considerable distortion, cracking, and faulting of the strata. Following these fractures, the open areas or breaks were filled with sediments either by the action of water or the movement of weak, clayey rocks. It is believed that clay veins or dikes were formed in this manner. The types of rocks that concern us as coal miners are generally sandstones, limestones, clays, and shales, otherwise known as "sandrock", "limerock", "fire clay", and "slate". A term to describe a structureless, lumpy, or nonlaminated impure shale is "clod" or "mudstone". An example of variations in mine roof formed by uneven earth pressures acting on strata of different degrees of strength is shown in Figure 4. When the sandstones are well cemented and are without definite bedding planes, they usually form excellent roof conditions, as in the case of the left side of the opening in Figure 4. Often it is difficult to determine if the sandstone is massive and the lower few feet may be quite irregular, and when formed by thin laminations as in the right side of Figure 4, the problem of support is increased, again evidence for the case of adopting and following a standard roof-support plan. Quite often, thin coal streaks will be developed near the base of large sandstones, representing a plane of weakness and can usually only be detected by drilling into the strata. When such coal streaks or "riders" are encountered, they can be expected to occur elsewhere at about the same level. Massive limestone is seldom encountered above coalbeds, but, instead, usually the roof will consist of thin, irregular shaped layers of limestone with layers of soft, limy shales contacting the coalbed, as in the case of the Pittsburgh coalbed. (See Figure 5.) The intervening shales are clayey and very weak, with the upper portion usually containing irregular shaped bodies of limestone. Experience in mining this coal and supporting this particular roof indicates that the roof strata is not continuous and definitely not predictable or trustworthy. Shales are of many different kinds, such as sandy, silty, limy, clayey, irregular, even-bedded, or thick or thin-bedded, and present the most variable conditions for support, whether with conventional wooden timbers or bolts. When the roof shales are uniform in composition, with thin laminations, a multi-ply beam can be formed by bolting the layers together similar to the manufacture of plywood. If the roof is not bolted immediately after exposure, the bolts can and do draw the layers back together, but the unit is not as firmly bonded as before separation. When "slickensided" formations or parallel shear planes cut through laminated shales, it usually cannot be safely supported by bolts, and crossbars are usually installed. One feature of even-bedded deposits of shales is the presence of intersecting joint planes, often extending for considerable distances. These joint planes are formed as the result of shrinkage of the shales during consolidation, often penetrating the strata from top to bottom at an almost ventical angle. When a mine heading coincides with a joint plane, parallel with it, the entire edge of the joint block may develop a cantilever action, and the roof will be impossible to support. Possibly, the most common type of shale mine roof is one with A modification of the shale roof that poses one of the most difficult control problems is a strata that contains clay, clay shale, or coal streaks. The coal streaks or coaly shales overlying the regular coalbed are incompletely developed beds and are generally without a definite structure, vary considerably in thickness, and have very little strength. Shales with a high clay content or because of previous leaching are often called "rotten" or "tender" since they disintegrate when exposed to the atmosphere. The clayey roof shale usually falls out around the supports, proving adequate anchorage for the bolts and failure of the rock. Visual Inspection Visual inspection of the roof is one of the important features of roof control. By this procedure, mining personnel can determine, to a limited degree, the nature of the roof surface and any changes that might be developing since the coal was removed. Visual inspection of the roof should be performed in an unhurried manner, |