Chapter 2.
Sequence of Geologic Events in New England and North America.
We have already explained a bit of the Science of Geology, but, in order to further orient the reader, we will here give a brief history of the major geologic events in temporal sequence. We have mentioned that we can talk with some accuracy of only the latest small fraction of the history of the earth—the last 500 million years [my]. What is now the eastern side of North America was then in the tropics near the equator, and the local region was ocean bottom. That ocean was populated by the first creatures to leave hard shell fossils including the trilobites. The coastline then was where the Adirondacks are now. The Adirondacks are remnants of an orogeny, a mountain building event, that occurred about 1 billion years ago, in the Proterozoic Era, and was part of the construction of the super-continent that preceded Pangaea. That continent has been named Rhodinia. Later Rhodinia broke apart creating the ancestral Atlantic Ocean that was later closed when Pangaea formed. That orogeny is called the Grenville Orogeny, named for a town in Quebec. Orogenies that preceded the Grenville Orogeny have been identified successively further and further west on the continent. Each orogeny added to the growing landmass of North America. The ocean bottom sediments uplifted and metamorphosed in the Cambrian Period were eroded from the mountains created in the Grenville orogeny.
In the Cambrian, Ordovician and Silurian Periods of the Paleozoic Era the beginning of the formation of Pangaea was locally marked first by the docking of the Bronson Hill island arc of volcanoes, the Taconian Orogeny. This process lifted, compressed and metamorphosed the ocean bottom sediments beginning the construction of Vermont and the Appalachians. The Oliverian domes date from the Taconian Orogeny.
Hike: Velvet Rocks, Gile Mountain, Dartmouth Skiway
550-490 mya.
In the Cambrian, nothing lived on land. Shelled marine invertebrates dominated in the oceans. They had displaced the unshelled sea creatures of the Vendian Period, which ended the Proterozoic Era. Sea levels were the same as they are now.
490-442 mya.
In the Ordovician, sea levels dropped and an ice age began. It caused a mass extinction of some of the Cambrian critters and lasted into the Silurian Period. The first fish and land plants entered.
442-415 mya.
In the Silurian, sea levels rose to high levels after the ice melted, fish got bigger and insects appeared on land.
415-355 mya.
In the Devonian, another glaciation was associated with a drop in sea levels and another extinction. Trees, forests and seeds took the stage and amphibians crawled up on land to enjoy them. The Acadian Orogeny added Europe to North America, continued the construction of the Appalachians and the creation of the New Hampshire series plutons.
Hike: Mount Cardigan
355-290 mya.
In the Mississippian and Pennsylvanian, together the Carboniferous Period, all the coal deposits were laid down. Sea levels varied during the Period. Vast forests of primitive trees became those coal deposits.
290-250 mya.
In the Permian, the assembly of Pangaea and the Appalachians were finished with the Alleghenian Orogeny. Another glaciation occurred with the greatest mass extinction of all, so far. About 75% of living species vanished. reptiles and conifers appeared before the end of the Paleozoic. Throughout the western US during the Paleozoic a shallow sea covered the land most of the time.
250-66 mya.
The Mesozoic Era is divided into three Periods, which we will discuss in turn. North America was now in the Northern Hemisphere south of its present latitude. Early reptiles evolved into dinosaurs and eventually birds. Mammals made an appearance in the middle of the era and flowering plants at the end. No further glaciation occurred until the most recent ice age.
250-206 mya.
In the Triassic, sea levels rose and Pangaea began to break up and the southern Atlantic slowly opened. With the further break up of Pangaea, spreading basins opened in the Connecticut River Valley in Massachusetts, Connecticut and in New Jersey. Red sandstone deposits from this Period can be seen on I91, I84 and several places in New Jersey.
206-144 mya.
In the Jurassic, sea levels rose further. The White Mountain magma series plutons and volcanoes, including Mt. Ascutney, erupted through the thinned crust of the splitting continents. The Atlantic continued to widen.
Hike: Mount Ascutney
144-66 mya.
In the Cretaceous, the continental alignment approached that of the present day around the Atlantic. The Laramide Orogeny marked the beginning of the Rockies. The Cretaceous ended with the extinction of the dinosaurs and many other species due to a massive comet strike near the Yucatan peninsula, or due to massive volcanic eruptions that followed the comet hit.
66-2 mya.
The Cenozoic Era is mostly the Tertiary Period. The middle of the Tertiary has the Alpine and Himalayan Orogenies, and the end of the period has the beginning of the last ice age, which ended just 10,000 years ago. Mammals, birds and flowering plants dominated the biota. The continents were in their present places by the end of the Era.
Consider how much wider the Atlantic got during this 66 my. Ocean spreading from the mid-ocean ridges is about 5mm per year; 5cm per 10 yrs; 5m per 1,000 yrs; 5km per million yrs and, so, 330 km [about 200 miles] per 66my.
2my-now.
The Quaternary Period consists of the Pleistocene Epoch, the last ice age, and the Holocene Epoch, the last 10,000 years, human time.
