A Geological Interpretation and Retrospect
Cut off by the Hudson, Champlain, and St. Lawrence valleys, the New England section of North America is one of the most distinctly marked of all the many geographic regions of the continent. It presents a variety and detail of physical features paralleled only in northern Europe. This peculiarly varied surface of New England has been the result of a long combination of geological events.
The New England district has been more frequently and for a longer aggregate time above the level of the sea than any other part of the region south of the Great Lakes. This has resulted in the erosion of the unchanged later rocks, thereby exposing the deep-lying metamorphic and crystalline rocks. The topography and the consequent diversified areas of fertile soil have nowhere more completely controlled the history of a region than in New England. The site of the earliest settlements, and the later growth of industries and centers of population, have been determined and controlled in the most intimate way by the geological history.
To the visitor coming to New England from the Mohawk valley of central New York where the rock strata lie horizontal, or from the Appalachian region of Pennsylvania where there is a distinct order to the folded strata of the parallel mountain ridges, the varied scenery of New England presents a peculiar charm. Usually seen from the limited point of view of valley or lowland, it produces an impression of tumbled hills and rock ridges, of lakes and rivers,without order or system. Something of its charm lies in this element of the unexpected. But there is, perhaps, an even greater satisfaction and pleasure to be gained in seeing the country more discerningly as revealing a harmony and order of successive events through geologic time which have made it what it is.
The trained eye of the geologist or geographer, looking over the landscape of New England, sees it with a vision extending back into time long before history began. The sculptured forms of the hills and valleys speak to him of processes that have been going on through geologic time. From this point of view no one can intimately know and comprehend New England whose acquaintance with it does not extend back at least a few million years. It is worth while to attempt to see New England through such long-vision glasses.
Standing on a hilltop almost anywhere in New England on a clear day, and looking around at the horizon, one notices that the high-level surfaces of one hill after another, approach the plane of the circular skyline. It requires but little imagination to recognize in the successive hilltops the remains of a once even and continuous surface of what was once a great plain, from which the valleys of today have been carved out by the erosive action of flowing water. This is most easily recognized from a considerable elevation. Especially in the broad upland high level areas of western Massachusetts at Whitcombs Summit on the Mohawk Trail is this apparent.
Southern New England is, in truth, a `dissected upland,e gently slanting upward from sea level in the south and east, and rising, in northwestern Massachusetts, to elevations of several thousand feet.
About a score or more million years ago, in what geologists call the Cretaceous period, all of southern New England had been worn down by the eon-long erosion of water and atmosphere until it was for the most part a plain almost at sea level. Geographers call this a `peneplain,’ bearing the same relation to a plain as a peninsula does to an island.
This plain was not worn down evenly, because the rocky mass that goes to make up the crust of this region of the earth varies greatly in its resistance. Those isolated resistant masses of hard rock, like the Blue Hills, Mt. Wachusett, and Mt. Monadnock, are called in general by the modern geographer `monadnocks.’ South of the Blue Hills there are few of these in New England. Durfee Hill in north-central Rhode Island, the highest in that State, is one of these monadnocks which must be looked on as old mountain masses worn down to mere stumps. The Blue Hills have probably had removed from their slopes and summits, by the slow action of the water, thousands of feet of rock. Mt. Everett, and Greylock in the Berkshires, the White Mountains, Katandin, and the Green Mountain peaks are remnants of mountains once Alpine-high.
Since that time this land has been tilted until in the region of North Adams the whole area has been elevated a couple of thousand feet, and as this tilting went on new valleys have been worn by the water. Those of western New England are deeper because as the land has been more elevated, the rivers have cut more deeply. Some of these trenched valleys are quite canyon-like. The Deerfield valley is already, startling as it may seem, one fourth as deep as the Grand Canyon. The Naugatuck valley, near Waterbury, is a narrow, trenched valley which has been cut to a depth of five hundred feet below the general level of the surrounding country.
The breadth of these valleys has been determined by the relative resistance of the rocks to wear. The upper portion of the Housatonic valley in Massachusetts, where it is broad and generally known as the Berkshire valley, lies along a belt of weak limestones which have wasted away under the erosive action of the weather and water. The lower Housatonic in western Connecticut cuts through a region of hard crystalline rocks and here its sides are steep and bold. Through this region the Housatonic descends five hundred and sixty feet, affording waterpowers which recently have been largely utilized. The valley of Millers River, which enters the Connecticut river in northern Massachusetts, varies in width as it crosses belts of harder or weaker crystalline rocks. Between Athol and Orange, where the rocks are weak, the valley is wide; above and below where its course is through harder rocks, it has been able to wear only a narrow gorge.
The Connecticut river, from its source to northern Massachusetts, flows through a region of hard crystalline rocks in which it has during millions of years worn a long narrow valley. From northern Massachusetts southward to the Sound, a distance of ninety miles, the valley is from fifteen to eighteen miles in width. This increased breadth is not due to the greater size of the river but to the relative weakness of the rocks through which it flows. The rocks of the Connecticut Basin are unlike those of any other portion of New England. They consist of sandstones and shales of a reddish or brown color, due to small quantities of iron. The brown sandstone, so generally utilized some decades ago for the house fronts of respectability in both New York and Boston, is a Connecticut sandstone, for the most part quarried at Portland. The soils of the lower Connecticut valley have a prevailing reddish tinge because of the rocks from which they have been formed.
The Connecticut lowland is not primarily the valley of the Connecticut river, but consists of the wide open confluent valleys of a number of streams of which the Connecticut is the master. The lowland extends southward to New Haven, but the river at Middle-town turns eastward and flows through a narrow valley in the hard crystalline rocks of the eastern upland, entering the Sound at Say-brook. Seen from the margin of the upland, this Connecticut lowland appears a long, deep trough. The crystalline highlands mark the level at which the peneplain extended across the valley.
Originally the lowland was a geosyncline or down-folding of rock strata forming a trough which in Mesozoic time became filled with strata of relatively soft sandstone and shale. At the time these sediments were accumulating, the lowland was a great lake or estuary and on its broad mud flats reptilians of the time have left the so-called `bird tracks’ so numerous at Turners Falls, of which there is a most extraordinary collection in the Amherst College Museum. These softer rocks have been worn away by water action more rapidly than the harder rocks of the uplands to form the present broad trough of the Connecticut valley lowland.
The terraces which are so characteristic along the Connecticut and Merrimack valleys are also to be noted in the valleys of all the major streams of New England. They mark remaining portions of successive flood plains which the river formed as the result of changes in the level of the land. They are not, as Hitchcock supposed and as is still popularly believed, due in any considerable measure to glacial action in the Glacial time, but are the result of oscillations of the land level, successive subsidence and elevation.
The floor of the Connecticut valley lowland is relatively level,a peneplain of the second generation. From the valley floor stand out ridges and masses of hard trap rock like Mt. Tom, near Northampton, and the Hanging Hills of Meriden. All of them are steep-faced to the westward. They are fragments of faulted and tilted lava sheets .which were formed interstratified with the beds of shale and sandstone. Mt. Carmel, north of New Haven, is probably a volcanic `neck,’ the stump of the ancient volcano from which some of these lava sheets were poured forth. Near Meriden, on the slope of Lamentation Mountain, is a locality known as the `ash bed’ where may be seen the volcanic ash of one of these eruptions, now hardened into rock. Imbedded in it are fragments of scorea and `volcanic bombs,’ rounded masses which fell into the ash bed in a half-molten form.
There were three successive flows of lava of which the second was the greatest, having a thickness of 500 feet. Its uptilted fragments form the highest of the mountains of the Connecticut valley. These sheets of volcanic trap are interstratified with sheets of shale, sand-stone, and conglomerate, evidence that between the periods of volcanic action there was subsidence and deposition.
After this period further great movements in the earth’s crust resulted in the fracturing of these strata of lava and sandstone and tilting them to the angle at which they now lie. The softer shales and sandstones were worn away and the edges of the lava sheets left in high relief where they constitute the characteristic ridges and hills of the Connecticut valley extending from East and West Rocks near New Haven northward to Mt. Holyoke and Mt. Tom.
The region of Narragansett Bay is similar to the Connecticut valley, a geosynclinal down-folding in which during the Carboniferous period there was deposition. These Carboniferous strata extending up into Massachusetts contain some fossils and in Rhode Island considerable beds of coal which have at various times been exploited but have proved too hard to be of actual commercial value.
The Boston Basin is also a region in which are some stratified rocks still more ancient, probably chiefly Cambrian. Near Weymouth at Hayward’s Quarry is a famous locality where fossil trilobites, relatives of our modern horseshoe crabs, have been found, eighteen inches in length.
The Taconic and the Green Mountains are an extension of the Appalachian Mountain system, probably formed at the same time. Their folded and highly metamorphosed strata are probably Silurian or earlier. The upper portion of the Connecticut valley occupies a syncline in the metamorphosed schists. The course of the river, like that of most of the major streams in New England, was determined at the close of the Cretaceous period. The misconception which popularly exists that stream courses have been largely developed since the Glacial time is quite incorrect. Even the minor stream valleys in the hard crystallines of New England are of vastly greater age.
The ice sheet which covered New England during the Glacial time was thick enough to cover hills as well as valleys. In eastern Massachusetts it certainly was 2000 feet thick and its front lay out in the sea at least fifty miles to the east of Boston. In the successive advances and retreat of the edge of the ice sheet, it scraped up loose soil, wore down rock surfaces, deepened valleys, transported boulders, gravel and detritus, and left the country mantled with a sheet of glacial drift or till. It blocked stream courses and formed the many lakes and ponds which dot New England. Along its southern boundary it left great ridges of detritus, known as terminal moraines.
In southern New England there are three distinct terminal moraines that may be distinguished. The outer one lies along Nantucket, Marthas Vineyard, and the hills of Long Island. The second extends along Cape Cod, the Elizabeth Islands, and from Point Judith across southern Rhode Island and Connecticut. The third line stretches from the Manomet hills, below Plymouth, westward to the Hudson river. But the popular view that Nantucket, Marthas Vineyard, and Cape Cod are formed merely from such glacial detritus is incorrect. Professor Shaler first showed that underlying these glacial deposits there are strata of much earlier age, and at Gay Head on Marthas Vineyard the highly colored strata which give the name are as old as the Cretaceous.
Throughout eastern New England, from Maine to Connecticut, are characteristic lenticular hills, known as drumlins. These were probably formed under the ice sheet where it met with impediment in its progress, which caused it to deposit its detritus. Such drumlins are especially noted at Ipswich, Groton, Boston Harbor, and in the neighborhood of Pomfret. Near the margin of the ice sheet there were formed irregular hills of washed and partially stratified drift known as kames. The cliffs at Scituate are of this formation. Long ridges extending generally north to south, of glacial material without stratification, known as eskers, are found from Maine to Massachusetts. These mark the course of subglacial streams in the bed of which detritus washed from the glacier was deposited.
With the accompanying table it may be possible to briefly summarize what has happened in New England these last hundred million years or so. During the Paleozoic period, New England was a mountainous region of Alpine heights. About the Boston Basin, especially at Weymouth, we have a portion of its shore with the trilobites that inhabited its mud flats still preserved. The age-long action of frost and water wore away the mountains. During the Carboniferous period along the shores of Narragansett Bay, which then extended up into Massachusetts, were marshlands where grew a primitive vegetation of cycads and horsetails from which were formed the coal beds of Rhode Island. At the close of the Paleozoic that great earth contraction which formed the Appalachian wrinkles resulted in the Taconic and Green Mountains.
With the dawn of the Mesozoic there were down-foldings of the earth’s crust forming the long trough of the present Connecticut valley, accompanied by the uplift of neighboring areas. Sediments from the upland accumulated in the troughs. A somber vegetation, devoid of flowers, spread over the lands. Primitive reptiles and in-sects developed. On the mud flats of the Connecticut valley the dinosaur, the phytosaur, and the aetosaur left their tracks as they hopped or crawled. Ninety-nine species of these reptilians have been made out in these deposits which accumulated to a depth of 10,000 feet. The great accumulation of deposit washed from the surrounding highlands overloaded the earth’s crust and weakened it, so that volcanoes broke forth and lava-flows overspread the region. This occurred at three successive intervals.
At the close of the Jurassic, further earth movements resulted in the fracture and tilting of these interstratified beds of lava and sand-stones. During the following Cretaceous period these irregularities were worn down by the slow action of the elements until all southern New England was reduced to base level, forming a peneplain from which stood up monadnocks of the harder, more resistant rock. After the close of the Cretaceous period further earth contractions resulted in the tipping or tilting of this plain, bringing the level in western Massachusetts to an altitude of 2000 feet above sea level. In the following Tertiary period, mammals first made their appearance. The drainage from the higher slopes now began to wear out the deep river valleys as they are today. Not only the Connecticut and the Housatonic, but the smaller streams gradually had their courses determined for all time, and as the tilting gradually became accented, wore their way deeper into the rock. From the eastern brink of the Mohawk Trail, if one looks over the land, the level of the Cretaceous peneplain and the work that the Deerfield and other streams have accomplished since are apparent. Through this time, while the valleys were being carved out, the mastodon (see Northboro) and many other creatures that would startle one to encounter today, roamed over New England.