Form of the Quarries
"Although many quarries represent the fusion of several individual operations with depth varying greatly for different quarries and therefore depart notably from the ideal pattern, there is a typical "shape" that applies and a typical process that is used for the extraction of the stone. "
Quarry Shape and Orientation
Of the many lithotypes quarried commercially, slate quarries are the most distinctive given the stone's unique formation.
In all Pennsylvania districts it was customary to follow a simple quarry plan. Openings were approximately rectangular or rhomboidal, two sides being parallel to the cleavage strike and two parallel to the grain. Quarrymen spoke of the edges which paralleled the grain as the "sides" of the quarry, with the other two edges being called "back'' and ''front.'' On account of the general parallelism in the strikes of bedding and cleavage, the "back" and "front" were also commonly parallel to the bedding strike.
Image Source: Bill Bliss, Easton PA
The quarrymen designated the several levels or benches of quarries as ''pieces.'' A typical slate quarry, showing several levels or "pieces" (above). The smooth edges at the limits of the pit have been cut with a wire saw.
In all of the Pennsylvania districts the "sides" trended northwest, with "back" and "front" extending northeast due to the geology of the region. A unique exception was the Albion Bangor Quarry, where the grain in the quarry was diagonal to the ribbon resulting in a quarry that was not square but instead was a parallelogram. This was due to one of the walls being maintained parallel to the ribbon and the contiguous wall being parallel to the grain. Although unusual in shape, the decision to create a quarry of this shape was an economic one. By cutting parallel to the ribbon, a larger proportion of clear stock could be obtained than by having the ribbons cross the blocks diagonally, as they would had the orientation followed the more common rectangle.
A portion of the USGS geological quadrangle map for Wind Gap uses black lines to indicate the strike and dip of the underlying rock. Note the line identified by A and B, cutting across the Albion Bangor Quarry, which clearly shows an unusual orientation relative to those around it.
Image Source: Penn Pilot
The unique relationship of the "grain" and the "ribbon" within the Albion Bangor quarry resulted in a quarry that took the shape of a paralellagram and not the more common rectangle. Slate has two lines of breakability – cleavage and grain. To understand the terminology better CLICK HERE
Oliver Bowles provides a more in depth description of this in his book The Technology of Slate.
Depth Depended on Nature
Because of the unique folded way that the layers of stone were laid down in the region, quarries that were not far from each other were very different. In quarries where the beds were horizontal or flat lying ("run fast," as the quarrymen put it) the openings were usually large but shallow; however, where the beds steeply dipped, the quarries tended to have small openings that were deep. At Pen Argyl the beds approach the vertical, resulting in several quarries that were well over 500 feet deep. At Bangor, which was only about 2-1/4 miles away, the folding was around horizontal axes resulting in the bedding being significantly flatter. As such the openings had a larger surface area and generally did not exceed 250 feet at the deepest. At Slatington the depths were intermediate.
In the Peach Bottom district which was a significant distance from the Slate Belt district, bedding was not a controlling factor, and as a result the depth was controlled not as much by geology as by more desirable working factors, such as safety of walls and drainage. It should be obvious that hoisting time increased with depth and that shallowness had consequent advantages which may have overbalanced other disadvantages.
The two sections below clearly show the difference that bedding orientation can make. When the bedding orientation is laying flat or "running fast", like those in Bangor, the quarries tend to be larger openings that are shallower in depth. When the bedding orientation is vertical , like those of Pen Argyl, the openings are smaller and generally greater in depth.
quarry sections.pdfUse the + and - at the bottom of the image to zoom in or out and the scroll bars on the bottom and right side to move.
Image Source: Pennsylvania Geologic Society
Image Source: Bing Maps
The image above shows one of the waste piles of the American Bangor quarry from aloft. Although the quarry has been closed for several decades, the shape of the waste piles and the patterns in the landscape created from methods of removal from the quarry are still very evident. In this case the incline plane used to remove waste was run up the vertical path in the center of the image.
Waste
Waste reduction was the largest single problem of the slate industry. The waste that resulted from the act of quarrying the stone was always the first problem since it needed to be removed from the pit with the full understanding that the result had no financial benefit. In addition to the loss associated with the quarrying process, was that resulting from the milling as well. All material needed to be cut to specified dimensions which meant additional loss and together loss was estimated to be between 60 and 85 per cent of all the slate taken from the ground. Some of this loss was inevitable, being due partly to irregularities in cleavage, jointing, or some other structural feature, partly to sedimentary factors, such as "ribbons", "hard rolls", and "knotty" slate. Much was assigned to inadequate exploration in advance of quarrying, and this, along with poor mill practice were both well within human control. Through the years, methods of excavation improved, to a point where overall costs associated with waste were significantly reduced, the wire saw being the most effective tool to cut stone to millable shapes from the beginnning of the process.
The orientation of the folds through the district are the reason that quarries in some areas are deeper than others. Click on the image above to see it in a larger view.
Clearing the Overburden
One of the most significant reasons that quarries were not opened on a regular basis was the need to remove overburden from on top of the slate deposit. In order to start the process of opening a slate quarry there was usually a significant amount of work that needed to be be done to reach the rock from which the slate could be removed. The over burden was often removed by hand in the early days and then more commonly by steam shovel invented in 1839. To improve the process tracks were sometimes laid from the location of the shovel to a point distant enough to ensure the moved material would not be in the way. If a quarry was being opened because a previous quarry was no longer productive, the older quarry might serve as a dumping zone, but only if it was clear that the old quarry would not be reused.
In 1922 Oliver Bowles wrote in The Technology of Slate, that the overburden was often an issue:
Attention has been directed to the serious condition that exists in many quarries where waste heaps are placed too close to excavations. Overburden material commonly forms the nucleus of such heaps, showing that insufficient removal of overburden is responsible for them.
This issue of improper removal of overburden was most likely responsible for more than one of the cave-ins that sometimes happened.
Once the overburden was removed, it was not uncommon to remove between 10 and 20 feet of defective slate near the surface which was most often done with blasting. The removal of this low quality slate was common and the depths for removal could be as great as 50 or 60 feet depending on conditions.
The image below clearly shows crib work built to hold back the overburden. Situations like this were dangerous due to the possibility of landslides. As Behre points out though it was not uncommon to see this and was often the result of not having moved the overburden back far enough to begin with or having expanded the quarry hole to a point where the removed overburden became an issue.
Image Source: Pennsylvania Geologic Society
Image Source: Pennsylvania Geologic Society
Two images that show the massive volume of waste that was created, Many of the piles were over 100 feet high and in the end, altered the landscape of these communities forever. Although the quarries have closed, the waste piles still remain today.
Click on the image to the right to see it in a larger full screen view.
Image Source: