Nahant: A Geological History
By Barry Sidwell
People often say that Nahant is “as old as the hills”. Let’s think about that for a minute.
When you look around, you probably realize that the houses and roads and trees, even though they have been here for many years, are fairly recent additions to the landscape. But you might think that the hills and valleys and marshes and ponds must have been here “almost forever”; that the landscape is “As Old as the Hills”.
Actually, in a geologic sense, the hills and valleys are relatively recent, and are constantly changing. The “landscape”, is always being sculpted by the forces of erosion; moving water,snow and ice, wind, gravity, and chemical weathering. So, when you think about this, you might realize that the landscape features could be much, much younger than the rocks themselves. And this is especially true of Nahant and eastern New England region in general.
When you walk around Nahant, whether on the beaches or on the quiet, tree-shaded streets, it can be hard to imagine that the landscape was one covered in ice thousands of feet thick, or that the rocks under our feet were once a seething cauldron of molten lava. But it was. And there is also a tremendous span of time for which the geologic history is simply lost.
Ice Age Nahant
The landscape that we see around us throughout New England is primarily the product of glaciation. The gently rounded hills, the many ponds, lakes, and marshes, and the beaches with their variety of sand, gravel, cobbles, and larger rocks, are all the work of the glaciers which covered the region during the recent Ice Ages. The rocks themselves, that we find on the beaches or see in “ledges” (which geologists call “outcrops”) have a very different origin, however.
In fact, most of the visible landscape features of the Boston area are the product of glaciers which covered all of New England during the last Ice Age. This massive ice sheet, was about 2000 feet thick at the time of the “last Glacial Maximum” (about 18,000 years ago. By about 12,000 years ago, most of the glaciers had retreated, but the landscape was bleak and cold and treeless, like the tundra of Northern Canada today.
Since much of the world’s fresh water was still locked up in the great ice sheets to the North, the sea level was lower. Because sea level was lower, much of the area around Nahant, even out to Stellwagen Bank, and south to Nantucket was dry land, although it was covered by glaciers. And those glaciers were “bulldozing” millions of tons of rock, gravel and sand out to the shoreline, which was miles to the East of here. In fact that’s how Cape Cod and the Islands were “built”. When the Ice Age ended and sea level rose, they became islands.
Before the Glaciers
Jumping to 3,000,000 BP (3 MY BP) fundamentally changes everything. To begin with, the ice has not yet arrived. This is the Pliocene Epoch, and we are now entering the realm of Geologic Time. The shorelines as seen from our imaginary airplane would be noticeably different. If we were to have a spaceship or a satellite, we would see many big differences in the global picture. North America and South America are basically the same shapes as they are in our time, but the global sea level is higher, so the coastlines are different. The Americas are not connected by the Isthmus of Panama, so the Atlantic and Pacific oceans are connected through the body of water which will someday be the Gulf of Mexico. Most of the state of Florida is absent, represented by a vast area of shallow water with scattered islands, similar to today’s Bahamas. Elsewhere, many of the prominent features of North America today are recognizable.
The plants and animals of 3 MY BP would mostly be familiar to a modern visitor, but there are some big surprises. You would encounter camels, rhinoceros and elephants (or more precisely, their close relatives the mastodons and mammoths).
Pliocene Life
The plants and animals of 3 MY BP would mostly be familiar to a modern visitor, but there are some big surprises. You would encounter camels, rhinoceros and elephants (or more precisely, their close relatives the mastodons and mammoths).
Perhaps most surprising to the modern visitor would be the gigantic “Terror Birds”, which many paleontologists regard as the descendents of the dinosaurs.
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