USA > Massachusetts > Essex County > The story of Essex County, Volume I > Part 2
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After these New England Alps were elevated and had at least begun wearing down, the entire region underwent a peculiar twisting strain, not, apparently, so much large in amount as sudden. All the harder rocks thereupon developed the more or less parallel cracks that one still sees everywhere in the ledges. Of these joints, there are
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GEOLOGIC HISTORY
commonly three sets. One is nearly horizontal. Another trends north and south. A third, or sometimes one or two more, make angles from forty-five to ninety degrees with the other two. Not all appear or are equally developed at any one place. The effect is to break up the entire district into polyhedral blocks, inches to yards on an edge, that look sometimes like enormous crystals, and may or may not, according to their size and the direction of their faces, be to the convenience of the quarryman.
Ofttimes, moreover, there has been a sliding of one block over the next. The surfaces are smoothed; and one can sometimes make out by running his hand back and forth which way the faulting has gone.
New England, then, early in the Mesozoic, was a mountain coun- try, comparable, let us say, with the Sierra Nevadas now. What we have in these late days is only the roots of the old range, the rest long ago weathered off and washed away.
There was, from early in the Mesozoic, a considerable valley where now flows the Connecticut from Northfield down. Into this, as into all valleys, washed the soil of the uplands, to form deltas at the mouths of the side streams and a flood-plain flooring the main valley. There are the remains also of the mud bottoms of shallow lakes. These deposits are red, as befits the arid climate of the time, in marked contrast to the gray granites that border the area. Their northern tip is at Northfield, just at the steps of Dwight L. Moody's church. Their widest part, some twenty miles across, is in Southern Massachusetts and Northern Connecticut. The red sandstones and slates reach the Sound at New Haven. In the district around Turn- ers Falls-for the Mesozoic is the Age of Reptiles-are those dino- saur footprints, the most remarkable in the world.
During the deposition of these red slates and sandstones, came at intervals three surface flows of lava, the largest some five hundred feet thick. These, much faulted, now form the highly picturesque mountains of black "trap" or diabase that appear all through Cen- tral Massachusetts and Connecticut. Similar lava flows, except that they were mostly underground, form the Palisades of the Hudson and the trap ridges of Northern New Jersey. All these are of Triassic age.
In Eastern Massachusetts, this same lava has filled certain of those joint-cracks in the older rocks which had appeared earlier in the
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THE STORY OF ESSEX COUNTY
Triassic. The result is the familiar "dikes," a couple of inches to scores of feet in width, that appear almost anywhere in the hard rocks. For the most part, these dikes are nearly black. But some are green. Moreover, some are Paleozoic in age. In addition, the younger granites form light-colored dikes in the cracks of older ones and thus exhibit the relative ages of the two.
Along the coast of Essex County, where the granite comes to the sea, these Triassic dikes are especially conspicuous, sometimes a hundred on a mile of shore, more or less parallel, but not seldom with the later ones cutting across the earlier.
As is the way with lava freezing in a crack, the contraction on cooling thoroughly shatters the resulting rock, the Giants' Causeway of County Antrim, Ireland, being the conspicuous example. On a coast, therefore, the waves break up and wash away the dike-stone much faster than the country rock. The result is a deep indentation, like Rafe's Chasm at Magnolia, and the still more remarkable chasms at Marblehead, that look as if great saws had been cutting into granite cliffs.
With these diabase dikes, early in the Age of Reptiles, ends the making of the hard rocks of Essex County.
The hard rocks of Essex County being now complete, their his- tory continues with the landscape carved from them.
After the great Appalachian uplift that closed the Paleozoic Age in North America, the eastern part of the continent stood almost still for an uncommonly long time. The quiet time, in fact, lasted quite through the latter part of the Age of Reptiles, some millions of years, according to present-day reckoning.
No mountain range, without renewed upliftings, can endure thus long against the forces of erosion. Triassic New England, at the beginning of the Mesozoic, comparable, almost, with Switzerland, had by the end of the period, worn down to a flat country almost at sea level. Above this plain stood, however, certain peaks not yet worn down. Among these are the White Mountains and Katahdin, Monadnock and Wachusett, the Blue Hills south of Boston, Agamen- ticus in Maine, with many a smaller rock hill, known only locally.
One can stand on almost any hilltop in Southeastern New Eng- land and looking over the country note that all ridges except a few
MANCHESTER-HOUSE ISLAND
Courtesy of The Essex Institute
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THE STORY OF ESSEX COUNTY
higher peaks are at the same level. Present-day valleys have been cut below this. But imagine these valleys filled, and one sees with the mind's eye this ancient plain, Cretaceous in age, over which crawled and waddled the last of the land dinosaurs, over which flew the verita- ble dragons of the Age of Reptiles.
The matter is, however, somewhat complicated by the fact that the Cretaceous flat country, about the close of the period, was itself elevated, somewhat intermittently, a few hundred feet. Its seaward border, therefore, wore down to a new plain. The level hilltops of Essex survive from a later flat country than that which appears over most of New England. The higher Cretaceous level begins some- what east of Fitchburg and Worcester, and the level of the hilltops in Essex tends to merge with the valley floors of Worcester County. The Essex County plain was the habitat of the early mammals.
These two lowland plains, and some others, having been formed by wearing down the ancient New England Alps, the whole of South- ern New Hampshire and Vermont, and all of Massachusetts, Rhode Island, and Connecticut, where tilted bodily, early in the Age of Mam- mals. At Long Island Sound, the district is still about at sea level. Along the northern boundary of Massachusetts, the uplift is from 1,400 to 1,600 feet. North of Boston and in Southern New Hamp- shire, the rock hills near the sea rise commonly rather less than a hundred feet. Thence the ancient plain slopes upward to the north- west, the general rock level for the edge of Middlesex being not far from two hundred feet.
From the beginning of the Age of Mammals up to the advent of the human period in Europe, this ancient plain-for the several levels are so near together that all may be treated as one -- now tilted up toward the northwest, stood some hundreds of feet higher than it does now. Into it, what are mostly the present-day rivers cut such of the present-day valleys as are edged with rock -- which is to say, vir- tually all of them. But the coast line was out in Massachusetts Bay, fifty miles or so off the present shore, and the Merrimac River may have come straight down past Lowell to reach the sea at Boston. In fact, Boston Harbor is only the old valley of the Merrimac or some other river, now flooded by the ocean as the land has sunk below its old level.
In the same way, all the harbors of New England, Salem, Beverly, Gloucester, Lynn, and the rest, are each the drowned valley of a senile
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GEOLOGIC HISTORY
river, that once cut into the old lowland plain after its uplift and reached the ocean some miles beyond its present-day mouth. The rock hills of the ancient country that were near the old coast now appear as off-shore islands, or as peninsulas and capes. Each harbor still has the river that once made it entering its in-shore side. The borders of each harbor tend to run out to sea as a string of islands that were once the valley sides. The Maine coast shows all this to perfection, Penobscot Bay on the largest scale of all. But anywhere, at almost any little cove, is the same history written out.
The result of all this is a sort of four-storied landscape visible from the hilltops of Essex. The material is hard rock, less of it granite and diorite than most persons realize, for the igneous rocks, being hard, tend to hold up the hills and be seen. Most of these rocks are of the Coal Period.
The hard rocks, made into high mountains at the end of the Paleo- zoic, were reduced during the Mesozoic to a nearly level plain. This plain is now the region of the upland farms. Above it stand the old- est portion of the landscape, the still more ancient remnants of the New England Alps. After the up-tilting early in the Cenozoic, came the cutting of the present-day valleys, in which now are most of the large cities and all the seaports. Lowest of the four stories of the landscape is the part now under water, that is really only the sub- merged portion of the level next above. Mountain remnant, upland, valley, harbor, these four were carved out of the rock mass of New England substantially as they are now before the advent of the last Ice Age.
The Pleistocene Ice Age is taken to have begun some half-million years ago, when there were no men in North America, and the men of Europe and Asia were not so human as they are now. It need not have been more severe nor more prolonged than that other Ice Age which came at the end of the Palezoic, nor the still earlier one that ushered the Palezoic in. We simply know much more about it.
There were four separate advances of the ice. Between these were mild periods, during which the ice retreated off all New Eng- land and the climate differed little from that of today. Indeed, one interglacial epoch seems to have been so much milder than the present time that the ice-cap melted off Greenland. Only the last of the four
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THE STORY OF ESSEX COUNTY
incursions of the ice has left traces in New England. This ended, as nearly as can be made out, somewhere around 13,000 or 14,000 B. C. for Southern New England and 4,000 years later for northern.
The center of the ice-cap which involved New England was in Labrador; and from this point the ice flowed out in all directions, even northward, just as it does from the center of the Greenland ice- cap now. The flow, therefore, over Essex County was about south- east, slightly nearer south. The groves cut by the moving glacier have this direction. But the rocks of the district are too hard to take either grooves or scratches as do limestone and slate. Rock sur- faces hereabouts tend rather to be rounded, smoothed, and polished by the over-riding ice. Such surfaces, often also obscurely grooved and scratched, are to be seen almost anywhere that overlying soil has been stripped off too recently for the characteristic glaciated surface to disappear from weathering.
Over this rock surface lies almost everywhere the boulder-clay, till, or ground moraine, a fairly continuous sheet, often scores of feet thick. This contains some scratched pebbles and many faceted peb- bles and boulders, that have been dragged over the underlying rocks frozen fast in the glacial ice. Most stone walls in New England are built of these till boulders. One gets in the walls their characteristic shape, quite unlike rocks split from a quarry, or broken from a ledge, or water-worn by stream or waves; and one sees by the variety of material from what a wide range these boulders have been carried. They are strewn along all the way from a few yards up to twenty miles and more from their parent ledges, the last of them worn down to pebbles by grinding on the rocks beneath. Still larger boulders, up to the size of a small house, that were frozen into the mass of the ice or carried on its surface, are now perched aloft where the ice dropped them when it melted away. The Whale's Jaw near the Rockport-Gloucester line in the middle of Cape Ann is an example. Another large erratic is Ship Rock, in Peabody, estimated to weigh 2,200 tons and known to have been moved two-thirds of a mile.
The rest of this boulder-till is mingled pebbles, sand, and clay, quite unassorted as to fineness. A pinch of it, taken from close to a boulder, rubbed in the fingers, leaves the skin dusted with the finest rock flour. Clearly, no stream of water, no action of waves or cur- rents, can make such a deposit, but only frozen earth carried by mov-
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GEOLOGIC HISTORY
ing ice. Some till is, however, sandy, the water from melting ice having somewhat washed out the fine clay.
In certain spots, commonly of the general order of fifty to one hundred miles back of the farthest point reached by the ice-cap, this till sheet is thickened into peculiar lenticular hills, called "drumlins" or "whale-backs," one or two hundred feet high, seldom more than a half-mile long. Their shape, typically, is that of an inverted spoon- bowl, with the long axis parallel to the grooving of the ledges nearby, and the highest point somewhat up-stream from the center. Indi- vidual hills, however, depart widely from the type.
Sometimes these drumlins are twinned, single hills with two sum- mits. Often they come in groups, a half-dozen or more within as many miles. The islands of Boston Harbor belong to such an assemblage that numbers nearly two hundred in all. Where the waves have cut into them, they prove to be solid till, without ledges. But somewhat commonly, espescially with the smaller examples, there is a core of rock plastered over with the boulder-clay.
Curiously, in Essex County and thereabout, the glacial drumlins tend to be higher than the rock hills. Prospect Hill in Andover, a double drumlin, four hundred feet to the top of its main summit, is the highest point in the county. A mile east of this is another drumlin, Bos- ton Hill, hardly a score of feet lower. But the peak of the rock hill formed by the Andover granite, a mile to the west of Prospect, is only 360 feet above sea level.
There is, in fact, a somewhat definite line across country, from Nashua southward, through Marlborough and Providence, to Point Judith. West of this line most of the hills have been carved from hard rock and their tops mark the ancient plain to which the New England Alps were worn down by the end of the Cretaceous Age. East of this line, the topography has been built up of glacial drift, the highest hills tend to be till rather than rock, and the tops of such rock hills as there are outline a sea-level plain of Tertiary Age.
One must, then, when he reconstructs in imagination the ancient upland plain of Northeastern Massachusetts, imagine also all the drumlins removed. They date late in the Ice Age, long after the last of the dinosaurs and the first of the placental mammals.
Because these till drumlins contain much clay, they hold the water better than most soils. Therefore they commonly remain green in
Essex-2
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THE STORY OF ESSEX COUNTY
summer when grass elsewhere is brown. Because they are free from ledges, they are likely to be cultivated or sown to grass, while the rock hills grow up to woods. These characteristics, together with their regular, rounded profiles, such as a rock hill rarely has, make. drumlins easy to distinguish anywhere.
The Essex County drumlins belong to a somewhat vague group of something less than three hundred, of which not quite two hundred lie within the district itself. The group has its northern limit in the region of Kingston, New Hampshire, and its southern in Lynnfield and Peabody. East to west, the drumlins string across the entire county, thickest in the region that includes Amesbury, Merrimac. Newbury, Haverhill, and North Andover, with a goodly number also around Ipswich and Hamilton. Haverhill, alone, including Bradford. has twenty, all except one important enough to be named. But all of Cape Ann east of Essex has only two.
This Essex County group contains several drumlins that are per- fect examples of the type. Among these are Pigeon Hill in Rock- port; Hog Island, rising a hundred and twenty feet above the Essex marsh; Turner Hill in Ipswich, except that it is a little too round; Osgood Hill and others in North Andover; Tower Hill on the west side of Lawrence; Asylum Hill in Danvers; several near together in West Newbury, Bradford, Haverhill, and Methuen. Few towns of the county north of Salem lack presentable specimens.
The great terminal moraine which on this side of the continent marks the limit of the last advance of the Labrador Ice Cap, fol- lows the general course of the Ohio River, crosses Pennsylvania a little south of the New York boundary, to continue throughout Long Island. On Long Island the moraine is double. The outermost mem- ber occupies the center of the island, drops into the sea on the east end, and reappears on Block Island. The inner member, for much of the way close to the Sound, forms also Fishers Island, comes ashore at Watch Hill, and follows the Rhode Island coast, never more than three or four miles from the shore, till it disappears in Narragansett Bay. The main highway from Watch Hill to Wake- field follows the south edge of this moraine. Here is one of the best spots in North America to see what a terminal moraine is like.
The Labrador Ice Cap, as ice-caps are wont to do, splits, in South- eastern Massachusetts, into three separate lobes. The moraine of
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GEOLOGIC HISTORY
the westernmost forms the Elizabeth Islands and then follows the east shore of Buzzards Bay. A later moraine of the same lobe, not so easy to make out, crosses from ten miles south of Fall River to near Plymouth. The moraine of the central lobe is the hilly country of Cape Cod that lies east and west along Cape Cod Bay. Nantucket and Martha's Vineyard mark a further advance of the same ice- tongue. The moraine of the eastern lobe is under water, its higher portions making the fishing-banks that feed Provincetown. It was the middle one of these three lobes that overran Essex County.
The front of the ice-cap drew back slowly as the climate moder- ated, the average rate of retreat for Southern New England being some twenty to twenty-five years to the mile. But the ice-front some- times advanced, and often stood still for centuries. Where it paused, it tended to leave a "recessional moraine," which looks much like a terminal moraine, except that it is on a smaller scale and may be only a little patch, and contains nearly always more sand and gravel in proportion to till and large blocks. In fact, till may be quite wanting. The granite knob that forms outer Cape Ann held up the glacier at that point, and the melting ice dumped down an uncommon load of great blocks that is still a wild and desolate country, and a most pic- turesque and fascinating one, after three hundred years of settlement nearby.
Here, though on the smallest of scales, is a real glacial moraine, the easiest to locate and identify short of Rhode Island and Cape Cod. It records, apparently, an advanced position of the tip of that small ice tongue which, a century or two later, built the moraine at New- buryport along its northeast side.
Such a recessional moraine is the one that crosses Massachusetts from Fall River to Plymouth. Another, still vaguer and much inter- rupted, runs from Saco, passes Wells on the land side of the village, and ends-for such as can trace it at all-north of Epping. South of Epping, running toward Portsmouth, is another small patch, some six or seven miles long. There are still smaller bits of recessional moraine strung along from Portsmouth southwest, half way to the Massachu- setts line.
What seems to be the last outlier of this system of recessional moraines appears at Amesbury southwest of the village, is interrupted by the Merrimac, reappears on the south margin, and continues south-
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THE STORY OF ESSEX COUNTY
east close to the river through Newburyport almost to Newbury Old Town.
Nowhere is this a typical moraine, being much too sandy, and nowhere can it be easily made out. The land was low when it was formed and the ice front stood in the sea. Therefore, does the entire deposit suggest water more than ice. Moreover, the fact that the moraine trends northwest and southeast, in the direction of the ice flow instead of across it, proves the ridge to mark the side of an ice tongue, not its end like a proper moraine. It corresponds, therefore, to that portion of Cape Cod which lies between the elbow and the wrist, from Orleans to Truro, the same ice-tongue apparently which built Cape Cod having now retreated to the Merrimac. The ice, therefore, must have lain over Newburyport . and West Newbury. Indeed, there are hints of the moraine at the end of the glacial lobe from near Newbury Old Town and along the marshes of Parker River to Byfield.
There is, however, one small bit of characteristic glacial moraine in Essex County, in the middle of the island portion of Cape Ann, partly in Rockport but mostly in Gloucester, and more or less parallel to the railroad between the two cities, a half-mile to the north of it. Dogtown Common is on this moraine.
The earth's crust tends to be in hydrostatic equilibrium : mountain districts are high partly because the rocks under them are light. Con- sequently, as mountain masses weather off and wash out to sea, the region, relieved of its load, tends to rise. Mountains are worn down steadily. They are uplifted by fits and starts. The net result is that they remain at something the same elevation for long periods. Thus an old mountain country like New England consists of rocks that were once deep under ground.
Conversely, a sea floor, loaded with land sediments tends to sink. Land wash may pile up strata a mile thick and yet the water over them remain always about the same depth. In the Connecticut Val- ley, for example, in the time of the dinosaurs, a fault-block kept drop- ping down as the rivers kept loading it with their sediments, until twelve thousand feet of sand and mud and silt and gravel had piled up. Yet, millenium upon millenium, the water remained shallow enough for the dinosaurs to leave their tracks.
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GEOLOGIC HISTORY
In the same way, when an ice-cap forms, the added load tends to depress the crust under it. But the displaced material tends to flow slowly to the region just beyond the ice and to raise the level of the land there. Meanwhile, the glacial ice, a mile or two thick and a thousand miles or more across, has locked up on land a vast mass of what had been sea water. Therefore, the sea level everywhere on earth lowered two or three hundred feet during the last Ice Age.
When the ice-cap departed, the water which had made it went back to the ocean and raised the sea level. At somewhat the same time, the land that had been under the ice, freed from its load, started to come up again. But the lands that had lain beyond the ice-cap were now out of equilibrium and sank. Thus a wave of elevation invaded the lands that had been ice-locked. All this took thousands of years. Meanwhile, the ice-cap thickened and readvanced several times and all these processes tended to reverse.
The result of all this has been most complicated alterations of level for Essex County. In brief, the land, after sinking some one or two thousand feet from its highest position before the Ice Age came on, has now recovered some two or three hundred feet of its loss. There is, besides, reason to suspect a tilting of the New England coast, upward north of Portsmouth, downward south of Boston, with Essex County remaining in between little altered either way. But the adjustment has been slow and intermittent. It may not be complete even now.
Even so, such things as peat and tree stumps and old stone walls below the present tide level are to be taken cautiously as evidence of recent change. There are indeed such-at Nahant, in Lynn Harbor and the Saugus Marshes, at Manchester, Beverly, and Salem. But there are likely to be anywhere local changes in the run of the tides that alter somewhat the tide level. All loose deposits tend to shake together and to lower their surfaces locally without any real move- ments of the solid rock. Peat in particular, consolidates on its way to becoming coal. There seems, therefore, to be little evidence that will stand examination to show any change of level along the Essex shore within the last thousand years. At most, the subsidence can be only one or two feet a century.
But a thousand years is a short while; and supposed recent changes of level amount to only ten and fifteen feet. We do know
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THE STORY OF ESSEX COUNTY
that because New England has sunk since pre-glacial times, there is now a land topography at the bottom of Massachusetts Bay and har- bors along its coast. Because New England rose part way after the ice went off, we now have sea-floor clays of glacial age available for bricks.
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