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"The physical properties of slate have a direct influence on quarry methods. The ease of splitting, the direction of slaty cleavage, the direction of the grain, the position of the joints, and the dip of the beds are all factors in quarry problems. The quarryman learns to know his rock and this knowledge guides him in his choice of methods. "
"The physical properties of slate have a direct influence on quarry methods. The ease of splitting, the direction of slaty cleavage, the direction of the grain, the position of the joints, and the dip of the beds are all factors in quarry problems. The quarryman learns to know his rock and this knowledge guides him in his choice of methods. "
Oliver Bowles, The technology of Slate, 1922.
Due to the unique geology of the region, certain areas were better know for their roofing slate while other communities in the district were better know for their school blackboards. This difference was often based not on a company choice of what to make, but was almost exclusively dependent on the hardness of the slate in the ground. Soft slate which is fine grained is much more suitable for chalkboards. The soft vein of the Slate Belt was located in the northern part of the district and included the communities of Bangor, East Bangor, Pen Argyl, Danielsville, Slatington and Slatedale. This highly valued, soft slate came from a relatively small portion of the district (just over 20 miles long) which extended south west along the base of South Mountain. The smoothness, uniformity, permanence and attractiveness made the slate from this area the most recognized blackboard slate in the world. Most specifications for blackboards asked for clear stock which constituted only about 10 percent of the quarried slate in the soft vein district and amounted to only about 1 percent of the actual quarried tonnage. Much of the rest of it was used for what was known as structural slate.
Quarries such as the Chapman and Keystone were worked for their hard vein slate. While located just 15 miles southwest of Bangor, and only 7 miles from Danielsville, both Chapman Quarries and the Keystone Quarry just beside it, were both known for their roofing slate. Never used for blackboards and less frequently used for structure purposes, both of these quarries became recognized for their slate due to the high demand for roofing material. Chapman Quarries produced enough product that a railroad was built through Bath to service the quarry.
A portion of a map from Charles Behre's Book, Slate in Pennsylvania, shows the defining edge between the hard vein (pink stripes) in which Chapman Quarries is located, and the soft vein (blue and purple) to the north which produced the best blackboard slate in the world. Click on the map to see the source view.
Slate Runs
The map to the left shows perhaps the most detailed perspective of slate in one particular region of the district. Most general geologists are happy with the identification of the three standard members of the Martinsburg Formation consisting of Omp (Pen Argyl), Omr (Ramseyburg) and Omb (Bushkill). For the slaters however, it was not good enough. Charles Behre saw more subtle variations evident in letter codes found in section views in his book which included Ol, Oe, Of, Ow Os and Om . While the Martinsburg Formation through the Bangor section of the Slate Belt is generally identifed by the Pen Argyl member, a much closer view of the geological distribution of the area around Bangor using data from Charles Behre, shows five of the major "runs" which were subdivisions of the members, each producing a different type of rock. Note that the mapped quarries in the image all sit within the North Bangor and Bangor runs.
The map above is the original source of the colored sections lines shown in the small interactive map above. It can be opened in a larger view and can then be zoomed in by using the roller wheel of your mouse.
The Good and the Bad
The slate reserves of Pennsylvania extend through large parts of the southeastern quarter of the State, and in point of time, through parts of the pre-Cambrian and the later Cambrian, Ordovician, and Devonian periods of the Paleozoic Era.
The shale and slate formation, measures from a minimum exposure of 1,600 feet to an estimated maximum of 6,000 feet, but of this only a few hundred feet is commercial slate. In structure this formation consists of a succession of minor close folds, generally overturned northward, so that their axial planes have a general southerly dip, but usually the synclines so formed have a steeply inclined southern and a gently inclined northern limb. In places the fold is extremely close and the overturn so complete that its axial plane has a very low southeasterly dip. The cleavage dips southward at various angles, as low as 5°, pointing, like the curvature of cleavage and jointing, to a secondary movement. These folds vary greatly in width and their axes also pitch alternately east-southeast and west-northwest at angles ranging from 5° to 10°. They also bend laterally from north to south. These folds have been more or less truncated by surface erosion, and in places the cleavage foliation has been crushed and bent over to the south by the friction of the southward-moving ice sheet.3.
What's in a Name?
While the slate quarries of the district are unique, the names of many of the quarries and communities of the Slate Belt district are not. A large percentage of the earliest quarrymen in the district came from the British Isles having worked those slate quarries before emigrating. William Chapman, who started Chapman's Quarry was from Wales having worked in the famous Delabole Quarry in Cornwall England, a quarry name (Delabole) that also was used in the Slate Belt. In addition, many of the slaters of the region worked in Lord Penryn's quarry in Bangor which was in Caernarvonshire County, North Wales, bringing the name with them. Names were so directly connected because many felt that the district in Pennsylvania reminded them of the areas where they had worked before.
While Bangor was a name that came directly from another location, the name Pen Argyl was unique, although its roots were linked to Wales as well. Pen Argyl received its name from a Welsh word meaning "head" and from the Anglo-Saxon word "argylite" meaning slate rock.
Roseto which was another slate community in the district was largely made up of immigrants from Italy. While a major family from the community was named Roseto, the name of the town is similarly sourced since the majority of the people in the community came from the village of Roseto Valfortore in Italy.
Pennsylvania - This State produces more than half of the entire slate output of the country. The product of 1894 was valued at $1,620,158. Of this amount $1,380,430 is the value of 411,550 squares of roofing slate while the remainder is the value of milled Stock.
Pennsylvania - This State produces more than half of the entire slate output of the country. The product of 1894 was valued at $1,620,158. Of this amount $1,380,430 is the value of 411,550 squares of roofing slate while the remainder is the value of milled Stock.
The image to the right is a map (plan view) with a cross-section of that plan below it taken from the margin of the Geological Map of the Wind Gap Quadrangle created in 1990. The map represents the abandoned Theo. Whitesell quarry (filled with water) which is located 1-1/2 miles north of Stockertown. In the cross section on the bottom, as well as the plan on top, solid lines and dotted lines are representations of bedding (both existent and eroded) while the long dashed lines represent cleavage.
The dark boxes in the map above represent the location of slate quarries in the community of Slatington. Again, the unique geology dictated the placement of quarries and as such the patterns of quarries on the landscape were very distinct. Notice that they are all lined up in rows, ideally placed in relationship to the location of good quality slate. Image Source: Dale, Thomas Nelson. 1914. Slate in the United States. Washington, DC: Gov. Print. Off.
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