Terms Defined
"Two slate shingles can no more be exactly alike in texture than two leaves from the same tree. This is part of the charm of slate. A slate roof has an air of hand craftsmanship that has become the distinctive, and even the "distinguished" feature of the building. "
Pete Papay
Standard Committee Report on Roofing Slate Thicknesses, Knots and Knurls ... Oh My (2014)
For additional information about slate terminology click here.
Synclines and Anticlines
Sedimentary rocks are often compressed from the sides through what is loosely described as shrinking of the crust of the earth. This process has the well-recognized effect that the layers or bedding planes are wrinkled into "folds." These folds are described as "anticlines," if convex upward, or ''synclines,'' if convex downward. Their sides or slopes are called "limbs." The plane which divides equally the angle between the two limbs of a syncline or anticline is the "axial plane" (red in the image below). its intersection with any given bed is the "axial line" or "axis" of the fold. If the axial line goes downward in a certain direction, the fold is said to ''pitch'' in that direction. If the axial plane is horizontal or nearly so, the fold is said to be "recumbent."
Foliation Strike and Dip
Directed pressure on rocks containing platy minerals causes foliation. When describing a foliation it is useful to note its orientation in space, as strike and dip, or dip and dip direction.The strike and dip of a rock layer define its orientation at a particular place. Strike is the compass direction of a rock layer as it intersects with a horizontal plane (see figure 1); dip is the angle of steepest descent of the rock layer from the horizontal, measured at right angles to the strike.
Cleavage
The distinguishing characteristic of slate is its "fissility". Most types of rock may be split more easily in one direction than in another, but in no other type is this property so prominent as in slate. The plane of splitting, due to slate's fissility, is commonly called the cleavage plane. Cleavage is due to the parallel arrangement of mica flakes and the elongation of other mineral constituents all in the same general direction. The horizontal thrust which caused the folding of the original beds also caused this parallel arrangement of minerals; therefore the cleavage is not always the original direction of bedding, depending on the part of the fold being considered. At either the top or bottom of a fold the cleavage is commonly perpendicular, while on the sides of the folds it is roughly parallel to the bedding. Not all deposits of slate are as simple as the case just described, and it can not be definitely claimed that horizontal pressures are necessary for the formation of slate. Probably the only necessary conditions are pressures from two opposite directions on a clay stratum with a limited degree of freedom to flow or yield in some other direction, and this direction of yielding determines the cleavage.
Although ribboning puts visual interest into a slate roof, the difference in color or physical durability can be detrimental to roofing shingles.
Slaty Cleavage
Slaty cleavage is the structure which above all others differentiates slate from other rocks and gives it economic value, since by virtue of it the rock splits much more easily in one direction than in others. A true slate may be split into thin sheets having comparatively smooth and even surfaces. Some of the Pennsylvania slates can be split as thin as one-thirty-second of an inch, but such sheets are too thin for practical use. In the manufacture of black board slates, uniform, smooth slabs 4 by 6 feet or larger may be readily split to a thickness of 3/8 inch.
Slaty cleavage is caused by recrystallization and rearrangement of the constituent minerals of the original shale by intense metamorphism. The direction of the cleavage planes thus formed is governed by the direction of pressure. In the quarries of Pennsylvania, the most important slate-producing state, the slaty cleavage rarely coincides with bedding, and commonly crosses it nearly at right angles. Where slaty cleavage and bedding coincide, the split surface is usually more uneven than where they intersect, and such slates are, therefore, more suitable for mill stock than for roofing. Weathered slates have poorer cleavage and are less durable than those quarried at considerable depth. Quarrymen know that the best splitting can be obtained from freshly quarried rock, since slates can not be split readily after they have dried out. Repeated freezing and thawing also destroys the splitting quality. Many American slates have remarkably perfect and true cleavage, permitting the production of uniformly thin slabs of large size.
In general, cleavage planes are parallel to axial planes of the folds and to the regional structures and therefore their position is fairly uniform over large areas. Since all of the Pennsylvania slate districts are part of the Appalachian folds, the true folded mountains, known as the ridge and valley province and since the trend of those folds is northeastward, cleavage planes as a whole trend northeast in all the districts. In the Lehigh-Northampton district it changes regionally, steepening westward. Where at Bangor it averages about 15 ° , from Slatington west it is nearer 55° .
Texture
The term "texture" is commonly used by geologists and mineralogists in describing the state of aggregation of mineral components in rocks. This term in the slate trade however has come into a somewhat more restricted use in describing the character of the split surface. A slate which presents a rough cleavage surface when split in the usual way is said to have more "texture" than one that splits evenly. Architects often showed a preference for what they called "textural roofs." By using the rougher splitting slates in various sizes and thicknesses they obtain what is designated as "roof texture."
A graphic from Charles Behre's book Slate in Pennsylvania showing the synclines and anticlines around the Slatington Area. Click on the image to see it larger.
Ribbons
"Ribbons" is a term applied to comparatively narrow bands crossing slants at various angles. They represent minor beds having somewhat different composition from the main body of the slate. Some ribbons are not objectionable since they may contain no injurious constituents and may not have undesirable colors. In some of the Pennsylvania deposits, however, the ribbons are rich in carbonates that weather more readily than the siliceous constituents. Such ribbons in roofing slates can be permitted only on those parts of the slates that are not directly exposed to the weather. Much of the ribboned slate can be used to advantage for structural purposes or blackboards. Conspicuous color of the ribbon may render it undesirable to use the slate.
When exposed to the weather the ribbons in the "soft-vein" slate of Pennsylvania decompose more readily than the clear stock. In sidewalks made of ribboned slate, deep grooves have commonly resulted from the weathering of the ribbons. The ribbons in the "hard-vein" slate of Pennsylvania resist weathering.
Image Source: Penn Pilot
The practice of cutting or wedging the slate parallel to the grain makes for a marked orientation in quarry dimensions, recognizable in all of the Pennsylvania slate quarries. Since all are in the belt of the Appalachian folding, and since the axes of such folds tend northeast, the openings usually have one major dimension in the northeast direction, with the other being almost without exception at right angles (in the direction of the grain). This is very apparent is an aerial view of Slatington from 1938.
Strike and dip are not specific to slate but are general terms found in geology which allow a person to understand the direction and orientation of the primary layers of the feature in question.
The strike indicates the direction of the formation and is typically represented as a compass bearing. It's direction is determined by the seam of intersection between the actual bedding planes and a hypothetical horizontal plane. On a geologic map the strike is indicated by a line segment oriented parallel with the strike line.
The dip is defined in degrees from 0 to 90 and indicates the amount of rotation of the bedding planes out of horizontal. To determine the dip. a hypothetical vertical plane is intersected at a right angle to the horizontal plane and the degree of rotation is determined from below the plane