The Slate Geology of The Slate Belt
"On the southeast of the state, forming an east-northeasterly belt between Easton and Reading and beyond, are the pre-Cambrian gneisses of South Mountain, flanked and dotted over with strips of Lower Cambrian quartzite and sandstone. Northwest of this belt and parallel to it is a great Cambrian and Ordovician dolomite and limestone plain from 3 to 6 miles wide, under which the quartzite dips. Still farther northwest is a slightly hilly belt of Ordovician shales, grits, and roofing slates (the Martinsburg shale), from 6 to 8 miles wide. At the southeast these shales and slates overlie the limestone (quarried for cement), and at the northwest they dip under the Silurian conglomerate and sandstone of Blue Mountain. "
T. Nelson Dale, Slate Deposits of the United States, 1906.
The Ramseyburg (Omr)
This member (represented as medium green and found in the lower portion of the view at the top ) consists of medium to dark gray slate alternating, in part cyclically, with light to medium gray, thin to very thick bedded graywacke and graywacke siltstone. Slates in this member are generally thick bedded at the top and thinner bedded at the bottom of member. About 2,800 ft (853 m); thick. Quarried for slate in upper part of the member.
The Bushkill (Omb)
Medium to dark gray, laminated to thin bedded slate with thin beds of quartzose and graywacke siltstone and carbonaceous slate (represented as dark green and found in the lower portion of the view at the right ). Bed thicknesses do not exceed 6 in. (15 cm) throughout the member, and are generally less than 2 in. (5 cm). The member is about 4,000 ft (l,220 m) thick, but complex folding and faulting makes this figure an approximation. Formerly quarried for slate and known as the hard slate belt of Pennsylvania by quarrymen. 1
The Martinsburg Formation
Slate quarries exist in the Slate Belt because the correct geology is present. All of the quarries were dependent on the composition of the Martinsburg Formation and where in that formation the quarries existed, determined if they were in the soft belt or the hard belt. This formation is generally defined as a gray to dark gray, and infrequently tan and purple shale and slate. Of this formation however most geologists recognize three independent members (represented to the left and below by three different colors of green) consisting of the The Pen Argyl, the Ramseyburg and the Bushkill members each of which are unique.
The Pen Argyl (Omp)
This member consists of dark gray to grayish black, thin to thick evenly bedded slate (beds commonly more than 10 ft (3 m) thick) rhythmically interbedded with carbonaceous slate, sandy slate, and very fine to medium-grained graywacke. Quarried for slate and known as the soft slate belt of Pennsylvania to quarrymen, this member is probably more than 5,000 ft (l,574 m) thick. (Represented as light green in the view to the left, it should not to be confused by the predominant yellow.)
Image Source: US Geological Survey 1990
The top left map shows the area around Pen Argyl with the large gray island (waste dump) in the top right corner being the Jackson Bangor Seven. The lower view shows the area around Edelman and Belfast, the gray island at the center being the Belfast Quarry. In both cases notice that the quarries (identifiable by the waste pile islands of gray) sit almost exclusively within the green zones of the Martinsburg Formation. The upper view, with lighter green is the soft belt and lower darker green represents the hard belt.
Not Exclusive but still Unique
In character, the available slate throughout the district ranges from a dense, highly metamorphosed rock with undulating cleavage and little or no traces of the original bedding, similar to schist; to a relatively poorly consolidated, irregularly flaking clay slate, having a marked kaolin clayish odor and a tendency to part parallel to the bedding. Differences in age, structure and composition are in general related to differences in geographic distribution.
Although the Martinsburg Formation that contained the slate stretched for some distance, only certain communities were able to capitalize on the resource. Only in relatively isolated areas and in separated periods of the earth's history have rocks had the peculiar combination of original composition and degree of alteration by pressure needed to produce commercial slate. Since in a region of folded rocks, strata of a certain age are generally exposed at the surface in relatively small areas only, there is a further limiting geographic factor in the occurrence and accessibility of individual beds or groups of beds of workable slate. As such, any given region generally includes slate beds which are roughly uniform as to composition, structure, and geologic age. This makes it possible to classify the slate belts of Pennsylvania into several distinct districts. A slate district may be defined as a geographic unit in which the workable beds have a virtually continuous outcrop, have suffered essentially the same degree of metamorphism and are approximately the same age, continuous and outcrop.2
It is this statement above, made by Charles Behre, which best sums up the uniqueness of the three distinct members of the Martinsburg Formation. This fact is evident based on the location of quarries and the communities that were the most profitable from the quarrying and fabrication of materials. Communities such as Bangor and Pen Argyl did far better than the districts found much farther southwest, well beyond the communities of Slatington and Walnutville. Communities such as Lenhartsville and Albany had slate quarries but they were few and far between. Profits were not as significant because quarries such as the Focht had poor quality green slate, and most of the opening was in red, slightly gritty beds which were really good for crushed slate only.
Take careful notice that within each of Behre's 4 colored section drawings above, all of the quarries from each individual section (indicated by their names) are located within the same layers of the Martinsburg Formation . Each portion of the formation is unique based on the same categorization seen above, except there is even more unique separation indicated by the additional Ol, Oe, Of, Ow Os and Om indicated by the smaller black letters. Similar letters from each section represent similar elements within the formation. This again shows why the quarries are not uniformly located throughout the district but are clumped together in groups. All of their locations are based ideally on available good slate.
Sections of an Onion
By looking at William Morris' black and white line drawing below left, which represents a generalized example of the valley and ridge formation of the Alleghenies , it is easier to interpret the four colored sections shown above, however, the sections above show specific distributions of the Martinsburg Formation. These sections, taken from Charles Behre's Book Slate in Pennsylvania (p. 281). represent slices through the folded layers of the earth like those found on an onion. Blue Mountain, located on the left side of the sections above, would be represented as one of the raised ridges which pass across the center of Morris' schematic image below. The wavy lines shown in the sections are the remains of the softer folds which have degraded faster than those which make up Blue Mountain over time. Looking closely one can see that the wavy patterns in each individual section above neither match each other nor are they uniform within themselves. The Martinsburg Formation is the geological portion of the layers which contains the actual slate. The section drawings above indicate the location of the Matinsburg Formation using lettering codes of Om and Oms (shown in black).
ABOVE: William Morris' representation of a geologically early formation which eroded to create the current Alleghenies.
