Bedrock Geology

Of the many materials beneath our feet, bedrock is the substance that underlies all the soil and deposits of more loosely-associated material. Even this solid rock is comprised not of one single material, however, and all bedrock types once took a different form. The bedrock of any particular place is a direct product of its unique geologic history, including how the rock-building materials collected, what pressures compacted them into stone, and how they have since folded, split, and thrust into their current state. Just as we classify plants and animals into taxonomic groupings, we separate bedrock into type classifications called “formations.”

There are three formations found in Jericho: the Pinnacle Formation, the Underhill Formation, and the Greenstone Formation. All three formations are among the oldest now found at the surface in the greater region.

The Pinnacle Formation is made of a schistose greywacke, meaning that it was created from bits of rock, mud, and debris that had already broken down somewhat from their original state. It is gray to buff in color, and you can commonly see “stripes” of varying colors in layers of the rock. Minerals present in the rock are quartz, sericite, biotite, and chlorite. This formation dates back at least to the Cambrian Period, 500 to 630 million years ago. It formed by sediments collecting at the bottom of an ancient sea, stacking on top of each other, then compacting into rock during the same event that created the Green Mountains, the Taconic Orogeny.

The Underhill Formation is a silvery-green color, and it is this rock’s color that is said to be the namesake of the Green Mountains. The formation is a compilation of phyllite and schist, with the major minerals being a magnetite-bearing combination of chlorite, muscovite, and quartz. It, too, formed from sea-bottom sediments.

The Greenstone Formation is volcanic in origin. When freshly broken, it appears dark green in color. In mineral composition, it is an amphibolite. This formation is much less common in the Vermont landscape than the other two, and in Jericho it is found only in the northwest corner of the town.

As in all of Vermont, the rock formations in Jericho lie in north-south strips across the landscape. This is due to the folds of bedrock that were pushed together in an east-west direction as tectonic plates collided. In Jericho, the western seam where the Pinnacle formation meets the Underhill Formation coincides with the West Fletcher fault line, a place where this east-west pressure has caused the rocks to crack. Much of the hydrology of the area is dependent on this fault, since it provides a rather dramatic topography. This fault also correlates to local history, since the dramatic topography in turn forms fast-flowing waterfalls, ideal for mill sites. The map to the left shows the approximate location of this fault line. If you compare the line to the bedrock map above, you can see that the fault provides the boundary along which the two types of bedrock appear at the surface.