Schist, Granite, and Water

Schist, Granite, and Water. . , During your visit to Fort Knox, you will see dramatic evidence of hundreds of millions of years of geologic history., The site's bedrock, a rust-colored schist known as the Penobscot formation, is the rock that builders blasted and moved in order to dig the fort's foundation. Once they removed the schist, workers shaped it into smaller blocks and laid it in the embankment (or enrockment) that rises dramatically from the waterfront to the fort's main level. , The schist that you can see today was actually formed through a two-stage process. On the bottom of a vast sea, more than 500 million years ago, sand, silt, and clay accumulated off the edge of a continent. When that continent (or geologic plate) slowly collided with another continent about 400 million years ago, high heat and pressure melted, pushed, deformed, and folded the ocean bottom's deposits in a mountain-building event that created the bedrock beneath Fort Knox., Fort Knox's granite, which came mostly from nearby Mount Waldo in Frankfort, is much younger than the site's schist and formed in a completely different way. The granite began as molten magma deep inside the Earth. More than 370 million years ago, the magma was forced up toward the Earth's surface. Instead of breaking through the crust and erupting as a volcano, this magma slowly cooled and solidified deep inside the Earth. A series of glaciers gradually scraped off the earth covering the magma, which had formed into granite. The last glacier retreated about 12,000 years ago. , The Retreat of the Last Glacier , 14,000 years ago , About 21,000 years ago, a glacier approximately one mile thick covered all of Maine and the Gulf of Maine. This glacier extended to the continental shelf as far as George's Bank (about 85 miles off the New England coast). After melting for about 7,000 years, the glacier's edge receded close to the present location of Maine's coastline. , 13,000 years ago , The glacier's heavy weight pressed the land down, allowing the sea to flow inland approximately 60 miles from Maine's present coastline. This sea covered today's Penobscot Valley. The glacier continued to melt at the same time, adding more water to the system., 12,000 years ago , As the land was relieved of the glacier's weight, it rebounded upward and the sea retreated back to a position similar to Maine's present coastline. Rivers such as the Penobscot which had likely carved channels during previous glacial retreats, re-established their routes to the ocean., 11,000 years ago , The land continued its rebound and pushed the sea out about 12 miles further than at present. Vegetation such as mosses, grasses, and sedges, appeared on the landscape, followed by shrubs and trees. This environment supported many types of animals including musk ox, caribou, and mammoth. The area's first humans, known as Paleo-Indians, migrated here to hunt these animals and use the land's resources., Present , With its evolving woodlands and forests, the land has continued slowly to rebound from the glacier's weight. At the same time, the rising sea level, caused by melting glacial and polar ice, has moved faster. The sea thus continues to encroach on the land, gradually changing today's coastline. As the sea has pushed inland and rivers have flowed seaward, large inlets like Penobscot Bay have been carved and shaped.