Outline of the Geology of Boston and Its Environs

By Nathaniel Southgate Shaler, S. D.
Professor of Palæontology in Harvard University

The topography, the soils, and other physical conditions of the region about Boston depend in a very intimate way upon the geological history of the district in which they lie. The physical history of this district is closely bound up with that of all eastern New England, so that it is necessary at the outset to premise some general statements concerning the geological conditions of the larger field before we can proceed to the description of the very limited one that particularly concerns us. In this statement we shall necessarily be restricted to the facts that have a special bearing upon the ground on which the life of the city has developed.

The New England section of North America — viz. the district cut off by the Hudson, Champlain, and St. Lawrence valleys — is one of the most distinctly marked of all the geographical regions of the continent. In it we find a character of surface decidedly contrasted with that of any other part of the United States. While in the other districts of this country the soil and the contour of the surface are characterized by a prevailing uniformity of conditions, in this New England region we have a variety and detail of physical features that find their parallel only in certain parts of northern Europe, whence came the New England colonists. This peculiarly varied surface of New England depends upon certain combinations of geological events that hardly admit of a very brief description. The main elements of the history are, however, as follows: —

The New England district has been more frequently and perhaps 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 permitted the erosive forces to wear away the unchanged later rocks, thereby exposing over its surface the deep-lying metamorphic beds on whose masses the internal heat of the earth has exercised its diversifying effects. This irregular metamorphism brings about a great difference in the hardness of the rocks, causing them to wear down, by the action of the weather, at very different rates. Then the mountain-building forces — those that throw rocks out of their original horizontal positions into altitudes of the utmost variety — have worked on this ground more than they have upon any other region east of the Cordilleras of North America. Again, at successive times, and especially just before the human period, and possibly during its first stages in this country, the land was deeply buried beneath a sheet of ice. During the last glacial period, and perhaps frequently in the recurrent ice times, of which we find traces in the record of the rocks, the ice-sheet for long periods overtopped the highest of our existing hills, and ground away the rock-surface of the country as it crept onward to the sea. During the first stage of the last ice period this ice-sheet was certainly over two thousand feet thick in eastern Massachusetts, and its front lay in the sea at least fifty miles to the east of Boston. At this time the glacial border stretched from New York to the far north, in an ice-wall that lay far to the eastward of the present shore, hiding all traces of the land beneath its mass.

These successive ice-sheets rested on a surface of rock, already much varied by the metamorphism and dislocations to which it had been subjected. Owing to the fact that ice cuts more powerfully in the valleys than on the ridges, and more effectually on the soft than on the hard rocks, these ice-sheets carved this surface into an amazing variety of valleys, pits, and depressions. We get some idea of the irregularity of these rock-carvings from the fretted nature of the sea-coast over which the ice-sheets rode. When the last ice-sheet melted away, it left on the surface it had worn a layer of rubbish often a hundred feet or more in depth. As its retreat was not a rout, but was made in a measured way, it often built long irregular walls of waste along the lines where its march was delayed. When the ice-wall left the present shore-line, the land was depressed beneath the sea to a depth varying from about thirty feet along Long Island Sound to three or four hundred feet on the coast of Maine. The land slowly and by degrees recovered its position; but, as it rose, the sea for a time invaded the shore, washing over with its tides and waves the rubbish left by the ice-sheet, stripping the low hills and heaping the waste into the valleys. While this work was going on, the seas had not yet regained their shore-life, which had been driven away by the ice, and the forests had not yet recovered their power on the land; so the stratified deposits formed at this time contain no organic remains. At the close of this period, when the land had generally regained its old position in relation to the sea, there were several slight, irregular movements of the shore, — local risings and sinkings, each of a few feet in height. The last of these were accomplished in this locality not long before the advent of the European colonists; some trace of their action is still felt on the coast to the northward.

This brief synopsis of the varied geological history of New England will enable us to approach the similarly brief history of the Boston district.

Looking on a detailed map of southeastern New England, the reader will observe that Massachusetts Bay and Boston Harbor form a deep but rudely shaped re-entrant angle on the coast. If the map is geologically colored, he will perceive that around this deep bay there is a fringe of clay slates and conglomerates, or pudding-stones. Further away, making a great horse-shoe, one horn of which is at Cape Ann and the other at Cohasset, the curve, at its bottom near the Blue Hills, includes a mass of old granitic rocks. This peculiar order of the rocks that surround Boston is caused by the existence here of a deep structural mountain valley or synclinal, the central part of which is occupied by the harbor. Long after the formation of the Green Mountains, at the time just after the laying down of the coal-beds of the Carboniferous age, this eastern part of New England, and probably a considerable region since regained by the sea, was thrown into mountain folds. These mountains have by the frequent visitations of glacial periods been worn down to their foundations, so that there is little in the way of their original reliefs to be traced. They are principally marked in the attitudes of that part of their rocks that have escaped erosion. The Sharon and the Blue Hills are, however, the wasted remnants of a great anticlinal or ridge that bordered the Boston valley on the south side. The Waltham, Stoneham, and Cape Ann Bay granitic ridges made the mountain wall on its north side. Narragansett Bay and Boston Harbor are cut out in the softer rocks that were folded down between these mountain ridges. The lower part of the Merrimac valley is a mountain trough that has been similarly carved out, and there are others traceable still further to the northward. This mountain trough is very deep beneath Boston; a boring made at the gas-works to the depth of over sixteen hundred feet failed to penetrate through it. If we could restore the rocks that have been taken away by decay, these mountain folds would much exceed the existing Alleghanies in height.

Within the peninsula of Boston, the seat of the old town, these older rocks that were caught in the mountain folds do not come to the level of the sea. They are deeply covered by the waste of the glacial period. But in Roxbury, Dorchester, Somerville, Brookline, and many other adjacent towns, they are extensively exposed. They consist principally of clay-slates and conglomerates, — a mingled series, with a total thickness of from five to ten thousand feet.. The slates are generally fine-grained and flag-like in texture, their structure showing that they were laid down in a sea at some distance from the shore. The conglomerates were evidently laid down in the sea at points near the shore; and they are probably the pebble-waste resulting from a glacial period that occurred in the Cambrian age, or at a time when the recorded organic history of the earth was at its very beginning. These rocks represent a time when the waters of this shore were essentially destitute of organic life. In the whole section we have only about three hundred feet of beds among the lower layers that hold any remains of organic life; and these remains are limited to a few species of trilobites, that lived in the deep sea. From the slates and conglomerates of the Cambridge and Roxbury series the first quarried stones of this Colony were taken. The flagging-slates of Quincy, at the base of Squantum Neck, were perhaps the first that were extensively quarried. A large number of the old tombstones of this region were from these quarries. The next in use were the similar but less perfect slates of Cambridge and Somerville; and last to come into use were the conglomerates and granites, that require much greater skill and labor on the part of the quarryman to work them. [1]Cf. Shurtleff’s Desc. of Boston, p. 189. — Ed. At first the field-boulders supplied the stone for underpinning houses and other wall-work; so that the demand for gravestones was, during all the first and for most of the second century of the existence of the town, the only demand that led to the exploration of the quarry-rocks of this neighborhood. Indeed, we may say that the exploration of the excellent building and ornamental stones so abundant here has been barely begun within the last two decades.

Although the rocks of this vicinity are extensively intersected by dykes and veins, those agents that in other regions aid the gathering together of the precious metals, — no ore-bearing deposits have ever been found very near Boston. There is a story that a very thin lode of argentiferous. galena was opened some fifty years ago in the town of Woburn, about eight miles from Boston, out of which a trifling amount of silver was taken. But, unlike the most of the other settlers in this country, the Massachusetts colonists seem never to have had any interest in the search for precious metals, and we know of no efforts at precious metal-mining in the eastern part of this Commonwealth until we enter the present century. The craze for gold and silver, which seems almost inevitable in the life of the frontiersman, was unknown in the early days of New England. [2]Captain John Smith, speaking of his voyage on our coast in 1614, says he came “to take whales and make trials of a mine of gold and copper;” but he added the alternative, “if those … Continue reading

Although the general features of the topography of this district are determined by the disposition of the hard underlying rocks, the detail of all the surface is chiefly made by the position of the drift or glacial waste left here at the end of the last ice time, but much sorted and re-arranged by water action. If we could strip away the sheet of glacial and postglacial deposits from this region, we would about double the size of Boston Harbor and greatly simplify its form. All the islands save a few rocks, the peninsulas of Hull and Winthrop Head, indeed that of Boston proper would disappear; with them would go about all of Cambridge, Charlestown, Chelsea, Everett, Revere, a large part of Malden, Brighton, Brookline, and Quincy. Charles River, Mystic River, and Neponset River would become broad estuaries, running far up into the land.

The history of the making of these drift-beds is hard to decipher, and harder still to describe in a brief way. The following statement is only designed to give a very general outline of the events in this remarkable history.

After the ice had lain for an unknown period over this region, climatal changes caused it to shrink away slowly and by stages, until it disappeared altogether. As it disappeared it left a very deep mass of waste, which was distributed in an irregular way over the surface, at some places much deeper than at others. At many points this depth exceeded one hundred feet. As the surface of the land lay over one hundred feet below the present level in the district of Massachusetts Bay when the sea began to leave the shore, the sea had free access to this incoherent mass of debris, and began rapidly to wash it away. We can still see a part of this work of destruction of the glacial beds in the marine erosion going on about the islands and headlands in the harbor and bay. The same sort of work went on about the glacial beds, at the height of one hundred feet or more above the present tide-line. During this period of re-elevation, the greater part of the drift-deposits of the region about Boston was worked over by the water. Where the gravel happened to lie upon a ridge of rock that formed, as it were, a pedestal for it, it generally remained as an island above the surface of the water. As the land seems to have risen pretty rapidly when the ice-burden was taken off, — probably on account of this very relief from its load, — the sea did not have time to sweep away the whole of these islands of glacial waste. Many of them survive in the form of low, symmetrical bow-shaped hills. Parker’s Hill, Corey’s Hill, Aspinwall, and the other hills on the south side of Charles River, Powderhorn and other hills in Chelsea and Winthrop, are conspicuously beautiful specimens of this structure. Of this nature were also the three hills that occupied the peninsula of Boston, known as Sentry or Beacon, Fort, and Copp’s hills. Whenever an open cut is driven through these hills, we find in the center a solid mass of pebbles and clay, all confusedly intermingled, without any distinct trace of bedding. This mass, termed by geologists till, or boulder-clay, is the waste of the glacier, lying just where it dropped when the ice in which it was bedded ceased to move, and melted on the ground where it lay. All around these hills, with their central core of till, there are sheets of sand, clay, and gravel, which have been washed from the original mass, and worked over by the tides and rivers. This reworked boulder-clay constitutes by far the larger part of the dry lowland surface about Boston: all the flat-lands above the level of the swamps which lay about the base of the three principal hills of old Boston — lands on which the town first grew — were composed of the bedded sands and gravels derived from the waste of the old boulder-clay. These terraces of sand and gravel from the re-assorted boulder-clay make up by far the greater part of the low-lying arable lands of eastern Massachusetts; and of this nature are about all the lands first used for town-sites and tillage by the colonists, — notwithstanding the soil they afford is not as rich nor as enduring as the soils upon the unchanged boulder-clay. The reason these terrace deposits were the most sought for town-sites and cultivation is that they were the only tracts of land above the level of the swamps that were free from large boulders. Over all the unchanged drift these large boulders were originally so abundant that it was a very laborious work to clear the land for cultivation; but on these terraces of stratified drift there were never boulders enough to render them difficult of cultivation. The result was that the first colonists sought this class of lands. One of the advantages of the neighborhood of Boston was the large area of these terrace deposits found there. There was an area of fifteen or twenty thousand acres within seven or eight miles of the town that could have been quickly brought under the plough, and which was very extensively cultivated before the boulder-covered hills began to be tilled.

After the terrace-making period had passed away, owing to the rising of the land above the sea, there came a second advance of the glaciers, which had clung to the higher hills, and had not passed entirely away from the land. This second advance did not cover the land with ice; it only caused local glaciers to pour down the valleys. The Neponset, the Charles, and the Mystic valleys were filled by these river-like streams, which seem never to have attained as far seaward as the peninsula of Boston. This second advance of the ice seems to have been very temporary in its action, not having endured long enough to bring about any great changes. At about the time of its retreat, the last considerable change of line along these shores seems to have taken place. This movement was a subsidence of the land twenty feet or more below the former high-tide mark. This is shown by the remains of buried roots of trees, standing as they grew in the harbor and coast-lands about Boston. These have been found at two points on the shore of Cambridge, a little north of the west end of West Boston Bridge, and in Lynn harbor. Since this last sinking, the shore-line in this district shows no clear indications of change.

With the cessation of the disturbances of the glacial period and at the beginning of the present geological conditions, the last of the constructive changes of this coast began. Hitherto mechanical forces alone had done ¦their work on the geography of the region; henceforward, to the present day, organic life, driven away from the shore and land by the glacial period, again takes a share in the constructive work. This is still going on about us. The larger part of it is done by the littoral sea-weeds and the swamp grasses. Along the estuaries of the Saugus, Mystic, Charles, and Neponset rivers there are some thousands of acres of lands which have been recovered from the sea by these plants. The operation is in general as follows: The mud brought down by these streams, consisting in part of clay and in part of decomposed vegetable matter, derived from land and water plants, coats the sandy bottoms or under-water terraces. In this mud, even at considerable depths, eel-grass and some sea-weeds take root, and their stems make a dense jungle. In this grass more mud is gathered, and kept from the scouring action of the tide by being bound together by the roots and cemented by the organic matter. This mass slowly rises until it is bare at low-tide. Then our marsh-grasses creep in, and in their interlaced foliage the waste brought in by the tide is retained, and helps to raise the level of the swamp higher. The streams from the land bring out a certain amount of mud, which at high-tide is spread in a thin sheet over the surface of the low plain. Some devious channels are kept open by the strong scouring action of the tide, but the swamp rapidly gains a level but little lower than high-tide. Except when there is some chance deposit of mud or sand from the bluffs along its edges, these swamps are never lifted above high-tide mark, for the forces that build them work only below that level. Their effect upon the harbor of Boston has been disadvantageous. They have diminished the area of storage for the tide-water above the town, and thereby enfeebled the scouring power of the tidal currents. Except at the very highest tides, the Charles, Mystic, and Neponset rivers now pour their mud directly into the harbor, instead of unloading it upon the flats where these marshes have grown up. There are other forces at work to diminish the depth of water in the harbor. The score or more of islands that diversify its surface are all sources of waste, which the waves tend to scatter over the floor. For the first two hundred years after the settlement, the erosion of these islands was not prevented by sea-walls; and in this time the channels were doubtless much shoaled by river-waste. Just after the glacial period these channels were very deep. Borings made in the investigations for the new sewerage system showed that the channel at the mouth of the Neponset had been over one hundred feet deeper than at present, — the filling being the rearranged glacial drift brought there by just such processes as have recently shoaled the channels of the harbor.

The depth of this port has also been affected by the drifting in of sands along the shores contiguous to the northeast and southeast. When the sea surges along these shores, it drives a great deal of waste towards the harbor. A fortunate combination of geographical accidents has served to keep the harbor from utter destruction from this action. On the north side, whence comes the greater part of this drifting material, several pocket-like beaches have been formed, which catch the moving sands and pebbles in their pouches, and stop their further movement. But for these protections — at Marblehead Neck, Lynn, and Chelsea on the north, and Nantasket on the south — the inner harbor would hardly exist, since these lodgements contain enough waste to close it entirely. At Nantasket the beach is now full and no longer detains the accumulating sands, which are overflowing into the outer harbor; yet, as the rate of flow is slow, its effect is not likely to be immediately hurtful.

Of the ancient life of this district there is hardly a trace. The two great and conspicuous formations in the basin — the flags and conglomerates of the Roxbury series and the drift deposits of the last geological age — are both very barren in organic remains, for the reason that they are probably both the product of ice periods. The rocks older than the Roxbury series are too much changed to have preserved any trace of the organisms they may have once contained. In the rearranged drift there are some very interesting remains of buried forests that have not yet received from naturalists the attention they deserve. These buried trees lie at a considerable depth below low-tide mark, and are not exposed, except by the chance of the few excavations along the shore that penetrate to some depth below the water-line. When found, these trees seem all to be species of coniferous woods. The cone-bearing trees appear from this and other evidence to have been the first to remake the forests of this region, after the cessation of the last ice time. Even the larger animals that once inhabited this district — the moose, caribou, etc. — have left little trace of their occupation. It is rare, indeed, that a bone of their skeletons is found, except among the middens accumulated around the old camping-grounds of the aborigines.

On the extreme borders of the Boston basin there are extensive fossil-bearing strata. At Mansfield, on the south, which is just outside of this synclinal, and within the limits of the Rhode Island trough of the same nature, there is a broad section of the coal-measures exposed in some mines now unworked. These beds are extremely rich in fossil plants. At Gloucester there is a small deposit of beds, containing shells of mollusks that lived in the early part of the present period, that lie just above the high-tide mark. But neither of these interesting deposits extends into the limits of the Boston basin.

Although this basin has lost the greater part of its rocks by the wasting action of the glacial periods, it owes more to these events than to all the other forces that have affected its physical condition. To their action we must attribute the formation of the trough in which the harbor lies, the building of the peninsula occupied by the original town, and all the beautiful details of contour of the adjoining country. To them also, it owes the peculiarly favorable conditions of drainage afforded by the deep sandy soils that underlie the terraces where the greater part of the urban population has found its dwelling-place.


Winsor, Justin. “The memorial history of Boston : including Suffolk County, Massachusetts, 1630-1880,” 4 vols. Boston : Ticknor, 1880-81.


1Cf. Shurtleff’s Desc. of Boston, p. 189. — Ed.
2Captain John Smith, speaking of his voyage on our coast in 1614, says he came “to take whales and make trials of a mine of gold and copper;” but he added the alternative, “if those failed, fish and furs were then our refuge, to make ourselves savers,” — and so they proved. — Ed.

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