USA > Massachusetts > Middlesex County > Chelmsford > History of Chelmsford, Massachusetts > Part 67
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All the bedrocks of this region are very, very old, running back many millions of years. The limestone is earlier than the Cambrian period of the Palaeozoic age, and is probably Archaean, and other rocks may be equally old. The age of many of the other rocks is still unsettled, for they contain no fossils and have been greatly changed by heat and pressure. The granite belongs to the Carboniferous or coal period of the Palaeozoic, and nothing except a few dikes is more recent.
Mr. La Forge writes me: "In general, the rock which might be said to form the 'ground mass' of the geologic map of the area, the rock into which the other rocks are intruded, or upon which they were laid down, is a biotitic granitoid gneiss. It is in part certainly sedimentary and in part probably igneous, but it is not feasible to map the two parts separately, as they are thoroughly inter-layered, hence, presumably, the igneous parts were intruded into the sedimentary parts, which are, therefore, older. There is also another biotitic granitoid gneiss which is probably of purely igneous origin, and which may or may not be closely associated with the first mentioned gneiss. It, however, is more abundant in the towns farther south than Chelmsford, and is not extensively exposed in that town.
"The lenses of limestone seem to be closely associated with the sedimentary part of the biotitic gneiss, and to be probably of the same general age. They are, as you say, greatly meta- morphosed, the metamorphism being probably due in part to the intrusion of the igneous portion of the gneiss and in part to the great deformation that the rocks have suffered.
"Besides the general 'ground-mass' of gneiss, there are a number of strips and lenses of metamorphic sedimentary rock, the principal types being a biotitic sandstone or quartzite and a fibrolitic mica schist, both being more or less pyritic in many places and hence characteristically rusty on the outcrops. It is not yet certain whether these lenses and strips are of later age than the gneiss, and infolded or infaulted in it, or of the same age and interbedded with it, and their age is, at least at present, entirely unknown. The widest and principal strip extends through Littleton, Westford, and the northern part of Chelmsford into Lowell, and southeast of it are a number of others, even as far southeast as the northern part of Bedford.
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"Cutting the gneiss and schists in almost every outcrop are many sheets, stringers and lenses of pegmatite and of muscovite- biotite granite, which also occurs as larger lenticular or oval bodies in several places, and forms a great mass, occupying practically the whole area northwest of the railroad from Ayer to Lowell. It is evidently later, as you say, than the other rocks of the region, and Professor B. K. Emerson has adduced pretty fair evidence that it is of late Carboniferous age, in which he seems to be correct. There are no rocks of later age, so far as I know, except a few diabase dikes."
There are some exposures of this bedrock which are worthy of special mention. One of these is the bed of the Merrimack below the Pawtucket dam. Here the mica schist has developed marked slaty cleavage, so that Professor Crosby characterizes it as mica slate. The cleavage planes are nearly perpendicular, and the river has eroded enough to leave ledgy shores. At the Middlesex Street station of the B. & M. R. R., in Lowell, the cut under Chelmsford street is through mica schist, there a firm hard rock. In other places, it contains more mica, and is a rusty, crumbly, rotten stone. In Chelmsford Centre, many of the boulders in the stone walls are composed of a much contorted mica schist, a rather firm stone.
The ledges on Littleton Street, Chelmsford, and at the summit of Robin's hill are of gneiss, in which there is a great deal of feldspar, and there are many boulders of this material. On the south slope of Robin's hill, however, there is a soft pyritic schist which was formerly worked to a depth of fifty feet as a copperas mine.
On the northwest side of Robin's hill is a lens of the ancient limestone, and another lies north of Littleton street. At both places the old lime kilns and quarries are easily accessible. This limestone is highly crystalline, and a large number of different minerals have been found here. Among these are black serpentine, actinolite, magnesite and scapolite. One form of the latter has been called Chelmsfordite. Professor Hitchcock and Professor Crosby give good accounts of these interesting minerals.
Granite is an igneous rock, and its origin is closely related to the geological history of the region. "Granite is now regarded as the product of the slow cooling and crystallization of molten, glass-like matter at a dull-red heat-matter which contained superheated water, and was intruded from below into an overlying mass of rock of sufficient thickness not only to prevent its rapid cooling and its general extrusion at the surface, but also to resist its pressure by its own cohesion and powerfully to compress it by its own gravity. * * * * The overlying rock * mass which furnished so large a part of the pressure required to form granite has at many places been removed from it by erosive processes that operated through great stretches of time. Indeed, it is only by the removal of this mass that granite is anywhere
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naturally exposed. Although this mass may have measured thousands of feet in thickness, its former presence is at some places attested only by a thin capping on the granite or by frag- ments which the lacerating action of the intruding granite has incorporated into itself."
It seems strange to us that the surface of this region was once hundreds, if not thousands of feet higher than it is today, but the presence of the granite, and the metamorphic condition of the older rocks are geological indications of this. In the Permian period, which followed the Carboniferous and closed the Palaeozoic age, there was developed a very strong stress in the earth's crust. The lateral pressure was so intense that the rocks were forced slowly up from their former positions, folded, crumpled and broken, and mountains were formed, of unknown height. All through the long Mesozoic age, the forces of nature wore down these ancient hills and mountains, until, at the close of the age, in the Cretaceous period, the land was almost leveled. Only harder masses of rock like Robin's hill and Nobscot, Wachusett and Watatic, rose above the undulating surface of the so-called peneplain, whose surface was only slightly above the level of the sea. This must have been truly an age-long process, for these levelling agencies of nature work very slowly.
At the end of the Cretaceous period, before or at the beginning of the Cenozoic age, there came a vertical uplift, which raised the peneplain to the level of a plateau, and which again set the brooks and rivers to carving out and transporting the rock material to lower levels. The old peneplain gradually disappeared, leaving as its legacy the even skyline which you see from Robin's hill. Ridge after ridge, and hill after hill, except for an occasional harder peak, have the same long level summit-lines. Toward the end of the Tertiary period, the land surface was much as now, except that the valleys were deeper, the ledges rougher, and the hillsides more jagged and irregular than now. The vegetation at this time, though it left few relics of itself in New England, contained many modern genera of trees.
The present topography and most of the present soil have been profoundly influenced, and in many cases caused, by the great glacial invasion which came at the close of the Tertiary period. For various reasons, not yet fully understood, the summers changed in character so that snow and ice remained all summer in the ravines and valleys of the northern highlands, in constantly increasing quantities. These masses gradually became valley glaciers, which, from year to year, advanced further and further over the adjacent lowlands. As the summers became less and less effective in melting, and additional snow and ice kept forming each winter, the whole surface, even the tops of the White Mountains, finally became covered with one immense continental glacier. This mass of ice was not static, but, because * T. Nelson Dale, The Chief Commercial Granites of Massachusetts, New Hampshire and Rhode Island, Bulletin 354, U. S. Geol. Survey, pp. 9-10, 1908.
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of its own weight, uneven strains in the mass, and alternate freezing and thawing, and the pressure from huger outflowing masses in Canada, it flowed very slowly in a generally south- southeasterly direction across New England, finding its southern termini in the ocean, and on Long Island, Martha's Vineyard, and the gravel hills of Cape Cod.
This immense glacier carried along great quantities of rocks and all the original soil of the region, and worked them over like some great mill. In the lower ice were frozen thousands of rocks, which were pressed down on the underlying ledges by the immense weight above. Thus the projecting hilltops and ledges were rounded off by this tremendous attrition. The exposed ledges in Chelmsford still show this rounding, and where the ledge is hard or has not been long exposed to the weather, the scratches and grooves worn by the rocks passing over, may readily be seen. It is from these scratches and grooves that the general direction of movement has been determined.
When the ice-sheet passed over valleys and low places, it usually partly filled them with the ground-up soil and roughly- rounded boulders, often brought from many miles to the north- ward. The weight of the many thousand feet of ice above con- solidated this material into the hard-pan, or till, which underlies the surface soil in many places. This material which was pushed along under the glacier is called the ground moraine. It forms the strong rocky soil still found on the uplands. When the glacier melted, it deposited over this ground moraine most of the material it had been carrying in the ice above, so in many places this good soil was buried under coarser material. For some as yet unknown reason the ground moraine was in places heaped up under the moving glacier in large lenticular hills called drumlins, whose longer axes point in the direction of the general flow. There is a large group of these drumlins in Groton. In Lowell there are two very good examples in Fort hill and Belvidere hill.
Where the glacier halted for a time on the land, there was always piled up in front of it an irregular ridge of the materials that had been carried in or on the ice. This contained many large boulders, some of them unmodified by the ice. Such a formation is called a terminal moraine, and there is no example of it in Chelmsford.
When the ice retreated evenly from year to year without long halts at any one place, the material it had been carrying was left on the ground moraine beneath. Sometimes there was only a thin layer of this, and in other cases great masses, which have been deposited in irregular hillocks. The clay was washed away from this gravel by the water from the melting ice, and the stones are often as well rounded as shore pebbles. The irregular hillocks are called kames, and are of frequent occurrence in Chelmsford and elsewhere in eastern Massachusetts. Often the sand and gravel in them is in layers, as may be seen where they have been used for gravel-pits.
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Sometimes there are long, irregular ridges of nearly uniform height made of these water-worn materials. One of these ridges, or eskers, runs over five miles from North Chelmsford, crossing the Lowell road near Golden Cove, parallel with the old turnpike, thence running southward. Another lies between Chelmsford and Westford, and still another runs from the south part of Chelmsford down near the Carlisle pines. These were parts of a system of drainage by which the waters of the melting glacier escaped. Some of the streams flowed in ice-tunnels at the bottom of the glacier, sorting over the materials of the ground moraine, and carrying away the clay in suspension, the so-called "glacier milk." As the ice-sheet melted, the roofs fell in, bringing down a lot of surface debris, later augmented by tributary streams from the top of the glacier. When the ice melted further, the reduced stream slipped down to one side, leaving its former bed high and dry, as one of these eskers. Some of the drainage streams flowed on top of the ice sheet, with banks of ice on either shore. As they cut deeper, the rock material they found was assorted, much as by the sub-glacial streams, and they finally carved and melted their way down to the ground beneath the ice-sheet, leaving an esker ridge when the ice had disappeared.
In front of the melting ice, where the slope was not too steep, the streams from the glacier spread out in a broad network, carrying vast quantities of sand and gravel, and distributing them in layers over the surface, while the finer clay, which made the water milky, was carried in suspension to the sea. There were many shallow ponds at the time, where dams of glacial debris held back for a time the vast volumes of water. In these ponds the glacial streams made many broad deltas which sometimes filled the entire pond. In some such ways as these were formed the barren sand-plains so well known in this region. So, too, were formed many of the lower plains now occupied by swamps and wet woods.
Now and then, among the kames or in the sand-plains, a block of ice would be isolated from the main mass or buried in the gravel. When this melted, a depression was left in the gravel, sometimes dry today, sometimes containing a small pond. These pits are called kettle-holes or ice-block holes.
Besides causing this variety of land-forms and furnishing most of our soil, the glacier, with its transported materials, upset the whole system of drainage, so carefully developed during the forming of the peneplain. The most important change for this district was in the course of the Merrimack river, which, in the Tertiary period, flowed into Boston Harbor. The glacial debris choked the old channel and forced the river into an easterly course, and, where it fell over ledges the water powers of Lowell and Lawrence have been developed and cities have sprung up. Dams of glacial debris also made Heart pond and most of the Middlesex County ponds and swamps.
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The muck or peat in the swamps and wet woods has all been deposited from vegetable remains since the close of the glacial period. In some of these deposits, especially at North Chelmsford and near Forge pond, Westford, the early settlers dug good quantities of bog iron ore. Water with acids in it trickled through the soft mica schists and came into the bogs charged with iron. When it met the air, the iron crystallized and was deposited, its weight bringing it into fairly compact masses. Richer deposits elsewhere have driven these humble sources of iron ore into disuse.
QUOTATIONS FROM HITCHCOCK.
Hitchcock, in "Geology of Massachusetts," 1841, says: In the bed of the Merrimack, from Chelmsford to Newbury, is a hard slate approaching quartz rock, which I apprehend will answer nearly as well for a road stone as the slate around Boston associated with the graywacke. [Page 211.]
He mentions the scenery and cascades near the mouth of the Concord river, and the deep cut for the railroad through mica slate, (at the junction of Chelmsford and Westford streets); also the scenery at Pawtucket falls on the Merrimack: "The bottom of the stream is composed of rocks, whose ragged aspect is finely contrasted with the smooth water and beautiful banks extending several miles above the city." [Page 280.] On page 264, he notices the exceedingly beautiful landscape, with the graceful curve in the river half a mile south of the village of Tyngsborough, (at the Chelmsford line).
He mentions two or three beds of limestone, and says the simple minerals imbedded in this limestone are numerous and interesting. The most common and abundant mineral is scapolite. It occurs both crystallized and compact. *
The crystallized variety is most abundant at * *
Chelmsford * *
* The crystals are sometimes transparent, more commonly opaque and white, having begun to decompose. Sometimes the crystal exhibits the primary form, or a right square prism, acuminated by four planes set on the lateral planes. More commonly, how- ever, the lateral edges are slightly truncated. Some of these crystals are one, or even two inches in diameter; though, in such cases, generally imperfect. Often this mineral is compact, and the color either white or lilac red. This red color, however, occurs also in that which exhibits an aggregation of prisms. * *
* At Chelmsford, small masses of black serpentine occur in the limestone, and at Littleton also, of a lively green color. They occur also at Bolton, and form a good vert antique. Very delicate and beautiful amianthus is found in veins in the limestone, about two miles southwest of the center of Chelmsford. The fibres are sometimes two or three inches long, and resemble the finest and most beautiful while silk. [Page 564.]
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HISTORY OF CHELMSFORD
The spinelle of several colors has been found at Bolton, Littleton and Acton. At Bolton and Littleton, it is sometimes red, and forms a real Balass ruby. At the same places, it is blue and hair brown, forming the pleonaste. The latter has also been found at Chelmsford, as well as Brucite, or chondrodite; and also a beautiful wine coloured garnet. * *
Rose quartz. This differs only by a shade of color from the amethyst. It would be much esteemed in jewelry were it not that it is liable to fade. Yet a faded specimen may be in a measure restored by being placed for some time in a moist place. This variety occurs in Blandford, Chesterfield, Chester, Williamsburgh and Chelmsford, but at none of these places is it particularly beautiful. [Page 185.]
The granite range, extending from Cohasset and Quincy, through Randolph, Stoughton, Foxborough, &c., nearly to Rhode Island, affords much valuable stone for architectural purposes. * * *
The stone used in Boston under the name of Chelmsford granite is found in a range of this rock, not connected with the deposit that has been described above. * * * This rock is pure granite, with no hornblends; and, being homogeneous and compact in its texture, it furnishes an elegant stone. Good examples of it may be seen in the pillars of the United States Bank, and in the Market House in Boston. These were from Westford. [Page 148.]
He mentions, on page 604, quartz of a rose red color as found in Chelmsford, but is not certain that mica slate is its gangue, because he found it only in boulders, yet has little doubt that such is the fact.
The limestones in Chelmsford are magnesian, and most of them are loaded with earthy impurities.
The limestones of eastern Massachusetts are among the oldest on the globe. The crystaline dolomite of Chelmsford occurs in the midst of gneiss, and is frequently fetid, so as to give a strong odor when struck with a hammer. [Page 568.]
The naturalist will be gratified to learn that on digging over the earth on the bank of Merrimack river, to lay the foundation of the locks of the Middlesex Canal, pine cones and charcoal were found at the depth of twelve feet from the surface, in a sound and unimpaired state; specimens of which are deposited in the museum at Cambridge. A small horn was also found at nearly the same depth from the surface, supposed to be that of a cow of two or three years old. [Allen, page 74.]
Chelmsfordite is the same as Scapolite (Wernerite) and is a silicate of aluminum with calcium and sodium.
Colors, light :- white, grey, pale blue, greenish, or reddish. Streak, uncolored. Transparent to nearly opaque. Hardness, 5 to 6. Sp. Grav., 2.6 to 2.8. Clearage indistinct. Occurs in crystals, or massive, or sub-lamellar.
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THE BOTANY OF CHELMSFORD, BY C. H. KNOWLTON.
The plants of this region have been studied mostly by amateurs. The first of these was Rev. John Lewis Russell, pastor of the Unitarian Church at the Centre from 1839 to 1841. He was a corresponding member of the Boston Society of Natural History, and occasionally read and published papers in which Chelmsford plants were mentioned. He was especially interested in mosses. Dr. Charles Walter Swan of Lowell collected very extensively in Lowell and Chelmsford, and contributed much information for the Flora of Middlesex County, published by Messrs. Dame and Collins, in 1888. His collection is now part of the Yale Herbarium at New Haven, Conn. Emerson and Weed's "Our Trees, How to Know Them," published in 1908, contains many fine pictures of Chelmsford trees. These were photographed by Mr. Arthur I. Emerson, and appear in the book as half-tone engravings. My own collecting in Chelmsford was done from 1900 to 1903, with occasional visits since. I have a list of 500 species which are known to grow in the Town, and there are, doubtless, over 800 species in the region, including the ferns and their allies.
Most plants have certain preferences as to soil, moisture, shade and other conditions, and, therefore, may be grouped according to habitat in what are called plant societies. The Chelmsford plants, I find, naturally arrange themselves in six of these societies. These are dry woods, cleared upland, wet woods, meadows, sand-plains, and lastly, a group of cosmopolites, mainly of European origin, which grow anywhere and everywhere, and are called weeds. Most of the plants in the first five societies are native, and were probably growing here when the settlers came, though in different proportions than now. The area covered by moist and dry woods has been greatly reduced by clearing and by fires, while meadows, cleared upland and unwooded sand-plains are more extensive than originally. In general, the region belongs to the coastal belt of dry woods, sand-plains and meadows, and is different from the plateau region of Worcester County and the morainal region of Cape Cod, though all three have many species in common. In the following pages, after a general account of common and conspicuous trees and plants, I have added brief lists, giving the Latin names of other characteristic species.
The dry woods are composed mainly of white oak, with occasional trees of red, scarlet and black oak. In some places, the chestnut is the prevailing tree, and white pines are abundant. Poplars, butternuts, hickories, sassafras, a few hemlocks, beeches and hornbeams, and here and there an occasional red cedar or savin, are mixed with the oaks in some places. There are a few straggling shrubs in these woods and along their borders, especially the hazel, the huckleberry and the low blueberries. There is a little mountain laurel in remote places. There are numerous brakes, and in rocky places a number of more delicate
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ferns. The ground is carpeted with dry leaves, much the same, winter and summer. Here, in early spring, a few May flowers may be found by those who know where to look, but the best flower of these woods is the pink lady's slipper, which blooms in late May. The other plants in the undergrowth are mostly weedy or inconspicuous.
Smilacina racemosa (L.) Desf. Maianthemum canadense Desf. Anemone cylindrica Gray. Anemone virginiana L. Aqui- legia canadensis L. Saxifraga virginiensis Michx. Baptisia tinctoria (L.) R. Br. Desmodium Dillenii Darl. Desmodium marilandicum (L.) DC. Desmodium paniculatum (L.) DC. Ceanothus americanus L. Lechea villosa Ell. Gerardia pedicu- laria L. Viburnum acerifolium L. Prenanthes trifoliolata (Cass.) Fernald. Solidago bicolor L. Solidago caesia L.
Cleared upland in pastures and old fields gives a chance for such woody plants as the juniper, sweet fern, blackberries, the sumachs (of which there are three kinds, the staghorn, the smooth, and the dwarf), the woodbine and the poison ivy. This last flourishes wonderfully by Chelmsford roadsides, and often develops into a regular hedge above the stone walls. The barberries also like the old pastures and flourish there. On and around Robin's hill, there are many beautiful locust trees. These are not con- sidered native in Massachusetts, but were long ago introduced from further south. Many of the plants of the dry woods, accustomed to light shade, flourish equally well in the open. The first spring flowers on the upland are the little everlastings, or "pussy's toes," followed by the dainty bluets, the blue violets and wild strawberry blossoms. There is a little yellow flower like the strawberry, but smaller, which fills the drier fields and often crowds out the grass. This is the cinquefoil, of which there are several kinds, one of them with silvery leaves. Another beautiful early flower is the wood betony, which has fern-like, velvety leaves and a head of irregular yellow or reddish flowers. Robin's plantain is a late spring flower with plantain-like leaves, but the flowers are like bluish and purplish daisies. Later in the summer, there are the pink bell flowers of the dogbane, the coarse but fragrant milkweed, and numerous asters and golden-rods. In many of the pastures, there are immense clumps of garget, a fleshy broad-leaved plant, with long racemes of whitish flowers and purple berries. Newly seeded fields contain little but "English grass" and clover sowed in them by the farmers.
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