Aggregates Manager Digital Magazine
Issue link: http://read.dmtmag.com/i/485104
AGGREGATES MANAGER April 2015 40 by Bill Langer Bill Langer is a consulting research geologist who spent 41 years with the U.S. Geological Survey before starting his own business. He can be reached at Bill_Langer@hotmail.com "Stones that are very large, and which cannot with ease be removed whole [from land to be tilled], may be blown to pieces with gunpowder. They will be not only more handy for removing, but far better to put into walls. For the blow- ing of round stones will make some square and regular faces. They will often come cheaper in this way than if they were dug out of quarries." Samuel Deane, 1822, The New-England Farmer; or Georgical dictionary, p. 430 Up until the early 1800s, the stone quarries in Barre, Vt., were not quarries in the traditional sense. They were simply boulder fields or ledge rocks. Boulder fields were places containing many boulders that had been picked up and transported by glaciers, and deposited by the retreating ice. Ledge rocks are natural outcroppings of bedrock. The boulders were shaped using plugs and feathers or by blast- ing with gunpowder. Ledge rock was quite easily quarried by pry- ing off layers of rock with crowbars or by blasting. Commonly, only the top few feet of granite were mined in ledge rock quarries. But as demand for Barre Granite increased, quarries began to extend downward into higher quality, stronger rock, resulting in quarries with the more familiar shape of a large hole in the ground. The most difficult step in mining downward into fresh rock was obtaining a free face from which to work. A mass of rock 3- to 5-feet wide and as deep as the desired thickness of the quarried blocks had to be removed to provide the necessary working space. This commonly was accomplished by a method called channeling, whereby closely spaced holes were drilled along a straight line. The thin webs between the adjacent holes were removed with a flat chisel called a broaching tool, thus form- ing a narrow channel. Four such channels were made bounding the working space. The rock between the channels was removed to provide the working area. Although channeling was more costly than blasting, some of the rock between the channel cuts could be removed in block, thus minimizing waste. Mother Nature also provided the quarrymen with some special tools. Granite forms when molten rock cools thousands of feet beneath the land surface. The pressure of the overlying rock is "captured" in the rock as it cools. When erosion exposes the rock at the land surface, the pressure is released, and fractures form as the rock "relaxes." The rock is also subjected to regional stresses caused by plate tectonics, which causes other fractures or planes of weakness. These features, some obvious and some subtle, influence the directions in which a block of granite will split. Quarrymen with keen eyes can observe these weaknesses in the rock and use them to their advantage. An 'Old Timer' in Barre described the planes of weakness as follows: "We look for three kinds of grains in the rock. When it goes up and down we call it 'rift.' Crossways, like that, we call it 'drift.' 'Hardway' is a grain running at right angles to the other two, a bad one to work. *** Hell, I've talked long enough. I got to get my stone out now. Don't worry, I'll take it easy, all right. I'm old enough to know how to do that." "Old Timer," an interview as recorded by Roaldus Richmond, 1940, Federal Writers Project It took a combination of channel drilling and geology to mine granite in Barre. TALES FROM AN Old Timer Drilling holes in granite.