Geology

Geology

Overview

"Moraine." The word is carved in stone as if to make it a permanent part of the landscape. But the word itself speaks of the geologic processes responsible for the impermanence of the landscape of Cape Ann and Dogtown.

The most recent changes (over the past 400 years) have been at the hands of humans or at least the European settlers who inhabited the then-abandoned "Upper Parish" now known as Dogtown. Part of that human impact involved the quarrying granite from the bedrock laid down half a billion years ago when Cape Ann's batholithic granite cooled deep under a volcanic Pre-Cambrian microcontinent known among geologists as "Avalonia". After hundreds of millions of years of being squeezed and ripped by the forces of plate tectonics, this bedrock granite, exposed by eons of uplifting and erosion, was more recently scraped, shaped, and strewn with large and small sediments deposited by the retreating glacier of the last North American Ice Age 10,000 years (a geologic moment) ago. These scattered rocks and boulders - even the famous Whale's Jaw and Peter's Pulpit - continue to be broken, dissolved, and worn away by the continuing processes of weathering and erosion, which in time will undoubtedly erase the word "moraine" here carved in stone with such human hubris.

Human Impact on the Landscape

The presence of humans before the arrival of Europeans has left only faint archeological traces on Cape Ann. The population of Native Americans (likely of the Penacook tribe) along the "Squam" River was small and transient and passing through only in the warmer seasons for fishing – or to enjoy the beaches.

The first Europeans to visit Cape Ann (as early as the Vikings under Leif Erikson, or later among the British, Portuguese, and French), of course, had no geologic impact, interested as they were only in the 'landscape' beyond the shore. But by the early 1600's, those itinerant fishermen gave way to more permanent settlers; Pilgrims from Plymouth granted permission to establish the "First Parish" of the place they named Gloucester, where they hoped to learn how to harvest the sea and feed their starving brethren.

The virgin forest on this highland part of Cape Ann was seen by those first settlers as a valuable resource to be shared 'in common' to be cut for firewood and lumber for building and shipbuilding. Within 100 to 150 years, the forest on this area known simply as 'The Commons' was nearly gone. The land of the Commons was granted to families and the area became known as the 'Upper Parish'. They dug their cellar holes, built their houses (now long gone), and built the stone walls you still can see – in a vain attempt to clear fields for farming. The cellar holes and stone walls, along with the quarries, a few trails and roads, two reservoirs and a few dozen words carved on the rocks are all that is left to mark the brief presence of humans here, as ever-changing Mother Earth relentlessly reaffirms her sovereignty.

Laying the Bedrock

"Avalonia" was not the first name given to the land now known as Cape Ann, in earlier times known as Wagawam to some of the Native Americans, possibly as Vinland to the Viking Leif Erikson, as Beauport to the French explorer Samuel de Champlain, even as Tragabigzanda to the British adventurer John Smith, before its current name in honor of Princess Ann of 17th Century England.

But "Avalonia" is the name given by modern geologists to the first land forming the ground beneath your feet. Avalonia, according to this footnote to the theory of plate tectonics, was formed as a volcanic archipelago or island arc above a subduction zone below the so-called Iapetus Ocean, between the archaic continents of Gondwana and Laurasia around 600 million years ago. Subduction occurs when, as a result of new sea floor emerging along some mid-ocean ridge, the older opposite edge of the oceanic plate collides with another plate and is pushed back down into Earth's hot interior, generating plumes of rising hot magma. The volcanoes that emerge above such a subduction zone eventually rise up from the sea floor to create new islands, which as they expand, merge together to create larger and longer islands. Japan, Indonesia, and the eastern Caribbean Islands are examples on current maps of such volcanic archipelagos that have risen above the subduction zones along the convergent boundaries between oceanic plates.

Since Gloucester's granite first cooled deep in the batholiths below Avalonia, the slow steady slide of continents (or really, the plates on which they ride) resulted some 300 million years ago in the disappearance of the Iapetus Ocean as Avalonia was squeezed between the continents of Gondwana and Laurasia. This collision, which crumpled up the Appalachian Mountains (along with the matching Atlas Mts. of northwest Africa, the Scottish highlands, and the Kjolen Mountains of Norway), created the supercontinent known as Pangaea, with the Avalonian Terrane fused in the middle.

After the Triassic and Jurassic dinosaurs laid down their fossil remains in these conjoined continents and when Pangaea began to split apart, Avalonia, once caught and smashed in the middle, was then torn in two – one half remaining attached in the east to what is now Europe (forming most of the British Isles and parts of Northern Europe and the Iberian Peninsula), the other half sticking with the rest of Massachusetts, Maine, and the coastal provinces of Canada in the west.

So are you now standing on the volcanic mountaintops of Avalonia – on Mt. Avalon? Of course not. Half a billion years ago those archaean landforms rose like modern day Mt. Vesuvius or Mt. Fuji, to elevations a mile or two higher than your current altitude. Those miles of overlying rock have since been scraped away – even before the relatively recent arrival of the Ice Age glaciers – exposing the dense, economically and architecturally prized granite that so long ago cooled slowly deep in the interior of Avalonia's volcanoes. This bedrock granite, unlike the boulders strewn about in this glacial moraine, is now visible mostly along the coast – where waves and tides have washed them clean – and in the quarries of Rockport and northern Gloucester, as well as in the higher elevations of Gloucester, such as Red Rocks and Mt. Ann near Routes 128 and 133.

Ice Age Litter

The boulders (massive and minuscule) that lie scattered about in the Dogtown moraine are the sediments deposited here by the ebb and flow of the glaciers of the Pleistocene Era, a relatively recent geologic age. Glaciers form whenever and wherever more snow falls annually than can melt, accumulating tons of snow compacted into layers of ice thousands of feet, even miles, thick – way more than you shoveled out of the driveway, even after the Blizzard of '78!

High in the mountains, glaciers slide down the rock slopes, carving U-shaped valleys such as Yosemite Valley or Tuckerman's Ravine. But during the Pleistocene Ice Ages, from 2 million to 10 thousand years ago, a cycle of climate change brought colder global temperatures that allowed a succession of continent-sized glaciers to grow from the Arctic as far south as Cape Cod, Long Island, and the Great Lakes. During intervening warm spells, those glaciers shrank, or 'retreated'. In retreat, the glacier did not move backwards – they simply did not reach as far south into warmer latitudes.

Though not as fast as mountain or valley glaciers, the slow seaward slide of a massive continental glacier (such as the one that reached as far south as Cape Ann 10,000 years ago) would grind away the rock and soil beneath it, like sandpaper (really massive sandpaper) smoothing the material under it. The sediments - large and small - like sawdust stuck to the sandpaper, get 'plucked up' and frozen into the churning ice at the base of the glacier and carried along until that ice melts. More like a conveyor belt than a bulldozer, a glacier thus moves tons of sediment (or till) slowly towards its southern-most edge, or terminus. There, the till is deposited in a mound or ridge called a moraine, or more specifically, a terminal moraine. Long Island, Martha's Vineyard, and Nantucket are part of the terminal moraine of this North American glacier around 15,000 years ago. After it retreated some, the glacier paused long enough around 12,000 years ago to form the western 'arm' of Cape Cod and Rhode Island. Again the terminus of the retreating glacier (still perhaps two miles thick) remained long enough at this latitude to deposit a terminal moraine on top of the bedrock granite of Cape Ann. The sand, pebbles, stones, and huge boulders you see all around you form a layer as deep as 150 feet thick on top of this bedrock.

There are other interesting features in the Dogtown landscape that result from the retreating glacier. Pigeon Hill in Rockport is a mound of glacial till shaped by an advance of the glacier over the terminal moraine left at an earlier time, a landform known as a 'drumlin'. There are several low areas such as Briar Swamp and Granny Day's Swamp that may represent glacial 'kettles', depressions formed by huge isolated blocks of ice at the melting edge of the glacier. The ridges and valleys that cut across Cape Ann from northeast to southwest, including the valley filled by Cape Pond and by Alewife Creek and Babson Reservoir indicate that the glacier moved from northwest to southeast (leaving ridges of moraine till perpendicular to the glacier's motion). The direction of this motion is also recorded in scratches and grooves in the underlying granite found on the rocks just west of Babson Family Quarry at Halibut Point State Park and other points indicated on this map drawn by the 19th Century geologist Nathaniel Shaler.