From: Industrial Minerals: 200 years and Going Strong: D. Conrad and D. Vanacek, 1990; updated 2005 ( S. King) and 2016
Current slate production comes from the Cambrian St. Catherine and the Ordovician Poultney and Indian River formations, which are the major formations within the Taconic Allochthon in southeastern Vermont. The Vermont section of the Vermont-New York Slate mining district is a narrow belt located between the New York/Vermont state line and the western margin of the Bird Mountain thrust slice mapped by Shumaker in 1967. Cleavage in these beds is high angle, ranging from 30 to 90 degrees, and the rock is highly faulted and jointed. Unique in their color range, the slates range from red, unfading green and fading green (turns brown over time), as well as purple, and green and purple mottled.
As reported by Adams in 1845, some of the first slates to be quarried in the state came from the Devonian Littleton Formation, prominent in southeastern Vermont. Slate has also been quarried from the Devonian Northfield Formation. These dark grey to black slates, probably more aptly classified as phyllites, have not been commercially quarried since the early 1900s.
About ten quarries are currently producting slate from the Vermont-New York Slate Mining district. The quarrying and milling operations employ about 300 people in the area, ranging from small family businesses to the largest concern, Vermont Structural Slate Co. The latter runs the Eureka quarrying and milling operation in Fair Haven.
Essentially all the slate operations produce blocks by drilling and blasting followed by manual splitting. Although drilling and broaching and wire saw techniques have been employed at the quarries, the tendency of the slate to split along the cleavage and along the “grain” makes it difficult to quarry uniform blocks using these methods. Blocks are removed from the quarry by power shovel, crane or power boom, loaded into a bucket, and then lifted from the quarry and deposited at the mill intake area. Blocks are stockpiled in the fall for processing during the winter months when quarrying stops.
Different milling techniques are used for roofing slate than for flooring tile and flagging. Both are cut by diamond saw and split to desired thicknesses. Floor tiles are then run through a planer to create uniform thicknesses. All other slate products are considered “structural slate,” and included fireplace mantles, electrical panels and window sills.