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Layered Rocks of the Waterville Formation, Central Maine
The Waterville Formation is a rock unit that underlies a broad swath of central Maine, from Lagrange southwestward through Corinna and Pittsfield into Waterville and Manchester. It is primarily a dark gray slate or phyllite, alternating with delicate thin layers of light gray siltstone or sandstone. It is the thin light and dark layering that distinguishes the Waterville Formation from adjacent formations.
The rock has split along vertical surfaces, called cleavage surfaces, because the rock has a microscopic fabric, or grain, in that orientation. This feature, called "slaty cleavage" is what allows slate to be quarried and split into large sheets for flagstones or roofing. The rock at this particular outcrop would not be very easy to work with, however, because the layers are nearly perpendicular to the cleavage. Sheets of slate would undoubtedly crack along the layers into small blocky pieces. Notice from the photo that the cleavage runs through all the layers, but curves slightly as it crosses from one rock type into the other.
Originally published on the web as the March 2003 Site of the Month.
Last updated on October 6, 2005
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