The Great Valley
"The slates of Pennsylvania, aside from those of Lancaster County and the southeastern part of York County which are known as the Peach Bottom slate, occur chiefly in Northampton and Lehigh counties in a strip from 2 to 4 miles wide on the south side of Blue Mountain, extending from Delaware Water Gap in a westsouthwest direction to a point 4 miles west of Lehigh Gap, a distance of about 32 miles. The chief centers of the slate industry here are at Bangor and Slatington."
John M. Nickles, Bibliography of North American Geology, 1913
Mountain Valleys
The Great Valley, also called the Great Appalachian Valley or Great Valley Region is a massive land form which, ironic due to its name, is a significant feature of the Appalachian Mountains stretching all the way from Georgia to Maine and beyond. Similar in concept to the naming of the Appalachian Mountains, which are composed of a set of mountains that regionally all have their own names (The Alleghenies, The Poconos, The Berkshires, The Adirondacks), this "Great Valley" is actually made up of a chain of mountain valley lowlands which are uniquely identified including the Tennessee Valley, the Hudson River Valley and a region in Pennsylvania known as the Lehigh Valley, What differentiates the Lehigh Valley, along with valleys farther south, from the those of the northern region, is a geological designation known as "the ridge and valley" which is unique in its formation.
Slate is abundant throughout the Pennsylvania Slate Belt region but this did not mean that slate could be dug just anywhere. The unique geological circumstances which lead to slate being present in the region, is what made only specific locations ideal for quarries. And those quarries, even the ones right next to each other, could vary in appearance, depth, ease of quarrying and quality of stone.
The Allegheny Mountains
The image below from the book Physical Geography by William Morris offers a perspective on what created the ridge and valley region, which originally consisted of a series of massive folded layers that have eroded away over millions of years. The Allegheny Mountains, which are part of this ridge and valley formation, are actually the remnants of a much much higher range of mountains, the layers of which have slowly deteriorated at different rates, resulting in a series of extended spines that run southwest from the border of northwestern New Jersey all the way to the southern border of the Pennsylvania at Warfordsburg just where the eastward extended arm of West Virginia virtually touches the state.
The first southeastern lift of this region, is a continuous ridge known regionally as the Shawangunk Ridge in New York, Kittatinny Mountain in New Jersey, and Blue Mountain in Pennsylvania. Many of the gaps which break through this ridge appear to be the result of rivers such as the Schuylkill, the Lehigh and the Delaware but one noticeable standout is Wind Gap which has no river and happens to be situated right in the heart of the Slate Belt.
Earthquakes!
According to a chemist, geologist and longtime slate manufacturer Albert Masters of Pen Argyl, millions of years ago, thousands of tiny earthquakes shook the foundations of most of what would become the Slate Belt communities. The biggest quake occurred at the site of the former Valley Quarry in Pen Argyl. This quake most likely ran up through the gap in the Pen Argyl Mountain. Some of the slate beds there reportedly slipped some 60 feet, causing a huge fault. (A slippage of 20 feet is considered to be a lot, according to Masters.) The second largest quake was in the Delaware Water Gap, although there are thousands of tiny fractures in most area slate veins. Originally, the slate laid flat in beds in the ground. Then the ocean waters covered the slate for thousands of years, and when the water receded, plants and foliage sprung up as the layers in the slate. The plants were turned into ribbons from one-half inch to eight inches between the slate layers. Ribbons, according to Masters, contain carbon, are only native to the Wind Gap- Pen Argyl-Bangor area, where the land was submerged hundreds of times in the ocean waters.1
The initial view of the map to the left shows Wind Gap, which is the break between two sections of Blue Mountain caused by an earthquake. The unique geology that produced this ridge running southwest towards Slatington is also the cause of the Martinsburg Formation which provides the extensive slate beds that were so valuable to the region. South of the Blue Mountain ridgeline in the map below is a series of blue rectlinear marks representing lakes and ponds, several of which can easily be seen and indicate the location of many of the quarries.
Image Source: David Rumsey Historical Map Collection
A geological map from 1960 produced by the Topographic and Geologic Survey of the commonwealth of Pennsylvania showing the central eastern portion of Pennsylvania where the slate belt lies. The dark brown and blue band represents the hard continuous ridge of Blue Mountain, while the yellow orange banding is the Martinsburg Formation which is the source of the slate.
CLICK HERE to open this map up in full screen interactive mode.
Image Source: US Geological Survey 1990
This view shows the area around Pen Argyl. The large gray island at the top on the right edge are the Jackson Bangor Seven. Click on the image to see it in a larger view.
The Martinsburg Formation
Image Source: Pennsylvania Geological Survey
Within the layers that make up the ridge and valley section of the Slate Belt district, is a specific geology that gives the region its unique richness in slate. Looking at the image to the left, the purple sections represent the harder stone of the continuous ridge of Blue Mountain while the green areas (not the yellow which is glacial till) represent the softer stone of what is known as the Martinsburg Formation.
While this map is intended for the specific function of geology, the small islands of gray are an interesting element for those interested in slate. These gray areas, which represent the "dumps" or waste material from the quarries, provide two pieces of additional information. First they show the location of quarries (each represented as very small light blue spots) and the relationship of those quarries to specific elements of the Martinsburg Formation. In addition they also show the overall massive scale of the waste material that was piled to the sides of the quarries themselves.
Notice that all of the gray islands in both the above and below maps sit within the green areas, the ones in the top map being in the soft belt and the ones in the bottom map being in the hard belt.
1. The Pocono Record. 1975. “All You Want to Know about Slate,” August 26, 1975.