MIDDLEBURY — The massive railroad tunnel construction project in the downtown has dug deep into Middlebury’s bedrock, exposing a deep-seated geology that most of us rarely — if ever — think about. 
Turns out, these rocks that we can now see formed in a much different environment hundreds of millions of years ago and reveal an interesting story of tropical days and shifting plate tectonics. 
This is an elementary review of the type of rock that underlies Middlebury, including some of what workers are blasting through to build the railroad tunnel. 
The bedrock in the Middlebury area is mostly composed of limestone and marble that originally formed about 475 million years ago, although there is subtle variability within those two categories, according to Dave West, a geology professor at Middlebury College.
West explained the basics.
“Limestone is a sedimentary rock that usually forms on the ocean floor in warm tropical environments like the present-day Bahamas,” he said. In the Middlebury area these rocks are gray in color and some even contain visible fossils from this ancient marine environment. 
Compared to a rock-like granite, limestone is relatively soft and easily worn down over time. “That is one reason why we have relatively flat topography in the Champlain Valley as opposed to the Green Mountains or Adirondacks where harder rocks are present.”
Marble, a metamorphic rock, is essentially a cooked version of limestone, explains West. Although it is composed of the same mineral (calcite) and has a similar hardness as limestone, it is usually a white color. 
In the past, marble was used mostly for sculpture and building construction and it was an important aspect of Vermont’s early industrial heritage. However, modern industry now uses finely pulverized marble in the manufacturing of a wide variety of materials, including paper, paints, and plastics (think of the stone that comes out of the Omya quarry off Foote Street in Middlebury). 
Today, when walking across the downtown Middlebury footbridge toward the Historic Marble Works District, one will notice that the layers of rock are aligned in a north-south direction and tilted downward toward the east. West explains, “The flat layers of rock that were once on the bottom of the ocean got buried and pushed upward during ancient plate tectonic collisions.” 
Those tilted layers and pre-existing cracks can be useful for the blasting process in construction. 
“When blasting, the goal is to break apart the rock into small enough pieces that they can be more easily moved. However, those pre-existing cracks can also cause stability problems and retaining structures are needed,” West explained. 
“This rock has been here for millions of years and has a complicated history, and that may be why this construction is taking so long. They not only have to blast through and remove the rock, but also stabilize what remains.”