Hundreds of millions of years ago layers of Graywacke (a resistant sandstone) were created 8 to 10 thousand feet at the bottom of the ocean when cataclysmic landslides of volcanic sand slid down the sides of offshore volcanoes into deep oceanic trenches.
440 million years ago, during the continental shift, a volcanic arc of islands including the deep oceanic trench were forced up and over the existing continent forming the Taconic Mountain range. These mountains were taller than the Himalayas are today. Over millions of years the Taconic Mountains eroded away into what we know today as the Taconic Ridge and the Rensselaer Plateau. During the creation (orogeny) of the Taconics, the older and deeper Greywacke layer was shuffled to be above newer layers in the area we now know as the Rensselaer Plateau. During the last glaciation, the plateau acted as a island under the glacier diverting rich soils in the glacial runoff to the lower elevations.
The Rensselaer Plateau has the oldest rock south of the Mohawk Valley and East of the Hudson Valley in the New England area. The Plateau is older than the Berkshires, the Greens and White Mountains. However, the Adirondack Mountains are older by a billion years.
Greywacke has formed this unique geological formation but also presents a challenge: as a highly prized resource used in construction of hard road surfaces, Greywacke has been eagerly mined in Rensselaer County.
Most natural lakes and ponds on the plateau originated between 14,000 and 12,000 years ago. These lakes predominately supported low vegetation until trees migrated into the region. First to develop were spruce, balsam poplar, and balsam fir, followed by eastern hemlock, white pine and later deciduous trees such as maple, beech, birch and oak.
The boreal forests of the plateau contain the headwaters of seven watersheds. Four drain into the Hudson River below the Troy Dam. Three of the northeast sections drain into the Hoosick River then into the Hudson River in Stillwater. Other waters feed the Tomhannock Reservoir (the public water supply for much of the county).
Taconic Tectonics: Rootless Mountains, Lost Oceans, Crushed Coasts is a talk presented to RPA members by NYS Geologist Bill Kelly December 2010.