Terms Defined

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.

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.