USA > Massachusetts > Plymouth County > Hingham > History of the town of Hingham, Massachusetts, vol 1 > Part 6
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At length the disturbing action ceased, and the earth, which had been shaken from its foundations to its surface, and rent asunder in a thousand localities, once more became quiescent. The effect upon the area of the basin was great, for where the waters had for an immense period spread themselves over the surface, and under which conglomerates and slates had been laid down, dry land appeared.
How strange to reflect that in these three words is embraced a fact without which all the stupendous events that have been mentioned, occurring over millions of years, would have re- mained entirely unknown to mortal man; for with the waters covering the basin, where could a trace of its long history have been found ?
The rocks of the Boston Basin as they present themselves in Hingham will now be noticed. Unfortunately the non-occurrence
SQUIRREL HILL
GEOLOGY OF HINGHAM.
MAAP
OF THE DISTRICT BETWEEN
LINCOLN STREET AND BEALS COVE.
Prepared by W O. CROSBY
Scole finch - 330 fret of 20 rods
R
BAKER'S HILL
Explanation of Colors.
Granite ond
Melaphyr
$644
Conglomerafr and
Shale
State Serves
Flate aund Shale
DIker
BEAU'S C/O VE
SECTION N W FROM THE BASE OF SQUIRREL HILL
Strect
Lincoln
WEYMOUTH
4
A
A
D
0
. .
B
4
a
Street
170
47
4
A
4
4
6.3
Now Level
=
SECTION FROM BEAL STREET TO BEALS COVE The honzontal and vertical acales of the sechons are nearly equal
43.
The Geology of Hingham.
of fossils in any of them makes it impossible to determine defi- nitely their age. The fact of slates within half a dozen miles of the town containing trilobites, thus showing them to be primor- dial, has led reasonably to the view that a part at least of those in Hingham might be found to be also primordial. The super- position of the strata, however, and their inclination, as far as these can be studied at their exposures, militate against this view. Nevertheless, considering how much is hidden from obser- vation where the great body of slate lies, towards Weymouth River, and the disturbances to which the formations have been subjected, it is by no means to be regarded as settled that slate of primordial age does not exist in Hingham as in other parts of the basin. It cannot, however, be shown that any is found resting beneath the rocks of the Conglomerate Series. That which occurs alternating with the conglomerate must be regarded as of the same age as the conglomerate itself. The great body of slate referred to above, towards Weymouth River, seems by its dip, as far as this has been determined, to be superior to the strata of the conglomerate series, and therefore a later rock.
This slate, on the maps is designated separately from that of the conglomerate series, as belonging to the Slate Series.
The Conglomerate Series comprises Conglomerates, Sandstones, Slates, and Melaphyr, which have together a thickness of nearly one thousand feet. The great disturbances alluded to, by which all these rocks were rent asunder by faults, and forced into ap- proximately vertical positions, will be more clearly apparent by a glance at the maps than by hours of reading.
THE CONGLOMERATE SERIES.
Conglomerate is formed of pebbles or angular fragments and gravel derived from pre-existing formations, these being cemented together into a compact rock. Sometimes the enclosed masses. are of considerable dimensions, being several feet in diameter. When the enclosed stones are pebbles, that is, are rounded, the rock is called Pudding-Stone; when they are angular it is called Breccia. The pebbles or fragments vary much in character, - those of Petrosilex, Quartzite, Granite, and other rocks being often found in close juxtaposition. Such is the case with the conglom- erate of Hingham, as may be seen at almost any exposure. When the rock is found made up exclusively of fine material, small gravel, and sand, it becomes a sandstone, and as such occurs in Hingham alternating with the coarser portions.
Conglomerate is the predominant rock over considerable areas of the town. It presents itself prominently in the harbor, com- posing the strata of the islands known as Sarah's, Langlee's, and Ragged, and its walls face the water along the coast front of Melville Gardens. It crops out upon the surface in great abun-
44
History of Hingham.
dance over the hilly region between South and Elm streets, appear- ing near the former in cliffs of considerable altitude, and it forms, with the amygdaloidal melaphyr, a part of the shore rocks of Rocky Neck that border Weir River, east of Planter's Hill. It also occurs abundantly about and over the high lands contiguous to Huit's Cove.
Away from the coast and the islands in the harbor the most imposing exhibition of this rock may be found in a narrow, pri- vate road that runs from Beal Street towards Weymouth River, some distance north of the Hockley Lane. Soon after entering this road it turns towards the north, winding about the base of some exposures of the conglomerate which lie between it and Beal Street. Following the passage through low ground and through forest growth for the distance of about a quarter of a mile, there suddenly appear high cliffs of the rock partially ob- scured by trees, rising to the height of forty to fifty feet, and pre- senting the appearance of having been torn asunder by some convulsion of nature, large masses being found in the foreground. The rocks extend along the road and near it six to seven hundred feet. The exposure here is well worth visiting.
The conglomerate rocks of Hingham were originally deposited upon the more ancient rocks, perhaps much farther inland than is now apparent, and were worn away by the erosion of the sur- face in after ages. At some localities, however, a partial coating of the conglomerate may be seen upon the granite, occupying depressions in it, showing where it once rested probably in con- siderable beds.
CLAY SLATE, OR ARGILLITE, OF THE CONGLOM- ERATE SERIES.
The slate of the conglomerate series in Hingham occurs, as may be seen by the maps, quite abundantly in the northern parts of the town, alternating with the conglomerate. The color of these slates varies considerably, a portion being of the ordinary bluish shade, while other portions are red or reddish. Both these colors are found quite near each other in the same exposure, as in Hersey Street on the left side going from, and not far from, South Street.
The slate of the slate series will be mentioned after notice of melaphyr, which is included in the conglomerate series.
MELAPHYR.
The name Amygdaloid commonly applied to this rock was given because of the frequent occurrence in it of cavities filled with other minerals than those constituting its mass, which are often approxi-
1
45
The Geology of Hingham.
mately almond-shape in their outline. These cavities, how- ever, may be entirely wanting, when of course the name amyg- daloid loses its significance. Moreover, rocks of a different composition have sometimes the same amygdaloidal structure. The name now applied to the rock by geologists is Melaphyr, and nowhere does it present itself in its typical and varied char- acteristics more advantageously for observation and study than in Hingham. The composition is the same as that of Basalt, which has as its essential elements, augite, magnetite, and titaniferous iron, but often containing a triclinic feldspar and other minerals, - the only difference being apparently the result of a change of some of the constituents by decomposition. Here it is found beautifully amygdaloidal over extensive areas, the amygdules being filled with minerals of several species which are sometimes arranged in concentric bands, the most common being epidote, quartz, chlorite, and calcite. At one locality, on land bordering Huit's Cove, there is an exposure of melaphyr, forming an escarp- ment on the slope of a hill, which is quite dark in color and in portions free from amygdules, and where these occur they are of calcite. This is found in the immediate neighborhood of other melaphyr, full of amygdules containing the various minerals men- tioned as common in the rock.
The best exposures for the study of melaphyr may be found at the northeast part of the town along the shore of Rocky Neck, on the northeasterly slope of Squirrel Hill, Lincoln Street, and at Huit's Cove. At all these places the amygdaloidal rock is abun- dant, and specimens of much beauty can be easily obtained. In the amygdaloidal melaphyrs of Rocky Neck fine red jasper and yellowish white epidote occur, both in nodules and in veins.
CLAY SLATE OF THE SLATE SERIES.
This slate, which forms a great body resting with apparent con- formity over the rocks of the conglomerate series, has a thick- ness of over one thousand feet, and undoubtedly is spread, as indi- cated on the maps, over a great area of the town toward Weymouth River. Its exposures are, however, not numerous, as the drift of the glacial period covers it from observation. It shows itself on the border of Weymouth River at Beal's Cove, and also at Huit's Cove. At the south side of the latter it forms a point of land which extends into the water. Here it is well-jointed, and the lines of stratification are distinctly perceptible. The dip is west- erly, and the inclination about 60°. On the north shore of the cove it appears associated with conglomerate and melaphyr, and portions of it show clearly lines of cleavage which are not often manifest at the exposures of slate in Hingham.
In a region where the rock formations have experienced great disturbance, as in Hingham, the dip of the strata varies very
46
History of Hingham.
much at the several localities. In attempting to obtain this, it may be well to admonish the reader, if not a geologist, that in the case of slates and some other rocks, the true lines of deposition by no means correspond with the lines of cleavage. It is owing to the planes of the latter that the rock is serviceable for the uses to which it is put in the arts, as a roofing material, and for other purposes. This kind of cleavage is called SLATY CLEAVAGE, and it is unquestionably due to great lateral pressure of the material of which slates are composed, after its deposition. The fact of such pressure being exerted upon the strata beneath the surface is well-known, and experiments by Sedgwick, Tyndall, and Daubrée, upon clay and other substances, demonstrated that the effect of pressure was to produce lamination.
The writer has thought it well, before closing his remarks upon the rock exposures of the town, to suggest two excursions that may be made to advantage by students interested in them. One of these is through the northern portion of Hersey Street, from South Street to Elm Street. The rocks mentioned rest immedi- ately on or quite near the margin of the road, and may be seen without going any distance from it in the adjoining fields. Since the examination has been made there has been some change on the east side of the street by the erection of a building, and the covering over of a portion of the rocks near; but thus far none that will lessen interest in inspecting those yet undisturbed. The other excursion suggested is that of a visit to Rocky Neck and a walk along its shores, as promising more pleasure and instruction than can be found in any other locality.
HERSEY STREET.
This street, in its northern part, affords a good opportunity to observe a succession of the sedimentary rocks of Hingham with the intrusive trap which is found with them. In ascending the rising ground from South Street, there occurs, on the right side, about 240 feet from the commencement of the road and back from it, an exposure of Conglomerate. It shows itself quite near the house of Mr. Allen A. Lincoln. Its face is parallel with the side of the house and at right angles with the road. On the next estate, 60 feet beyond, there is rock exposure near and facing the street, the first part of which is composed of trap and constitutes a dike six or more feet in width. This is succeeded by conglom- `erate, with which it makes a close junction. This conglomerate extends about 15 feet and is followed by a reddish slate extend- ing 20 feet, in the centre of which is a second trap dike. Suc- ceeding the slate is more conglomerate, which shows itself 50 feet or more. There is no further exposure on the right side of the road for 1090 feet, and then it is found that the limit of the
GEOLOGY OF HINGHAM. MAP OF ROCKY NECK ON
SECTION I.
.
WEIR
WEIR RIVER.
Prepared by W.O. CROSBY. Scale 20 rods or 330 feet =1 inch B.Meisel, lith.
SECTION II :
RIVERE
5°-10
EXPLANATION OF COLORS
Granite
Conglomerate ( Series
Melaphyr
Conglomerate
Dikes.
SECTION HI
Faults
W
E
Sea
Level
SECTION I
N
S
Sea.
Level
Horizontal Scale 330'= 1"
Vertical Scale. 165' = 1"
2
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The Geology of Hingham.
sedimentary rocks has been passed, as granite now appears. This extends 30 feet and is followed by an exposure of trap. Beyond this trap, which here crosses the street, the rocks are all granite.
On the left side of the street, ascending the hill from South Street and about 310 feet from it, there is an exposure of rocks which present themselves in the following order : conglomerate nine feet, slate six feet, sandstone twenty feet, slate again twelve feet, this last being succeeded by a dike of trap about nine feet in width. Beyond this trap there is no exposure for about 60 feet, at which distance another ledge appears, the first part of which shows blue and red slate six feet, the rest of it being conglom- erate, which extends 36 feet. Another space, of 72 feet, without rock follows the conglomerate, when this rock reappears in another ledge, - composing the first part of it for six feet, the rest of it, 45 feet, being blue and red slate. Still another space of about 80 feet occurs without rock, when sandstone appears along the road for the very considerable distance of 110 feet. Trap, partially covered with soil, succeeds the sandstone for about 40 feet, then conglomerate with an exposure of six feet. Beyond this conglomerate, which is the last seen on the road of the sedi- mentary strata, no other rocks appear on the left side of it for 650 feet. Then appears a considerable elevation of trap rock, which extends along the street about 30 feet and back upon the adjoin- ing fields towards Elm Street. As stated above, when mention- ing the portion of this dike exposed on the right side of the road, there are no other rocks beyond it excepting granite.
ROCKY NECK.
East of Planter's Hill, and partially separated from it by a depression of the surface, is an elevation of land forming a prom- ontory, which is bordered by Weir River on its north and east- erly shores. The rocks here, finely exposed as they are along the water's edge, and exhibiting well their relation to each other, afford one of the localities the best worth visiting of any within the town. The map of course shows the development over and beneath the surface of the land as made known by the rock expos- ures ; but a statement of what may be readily observed in a walk along the margin of the water will perhaps help visitors to understand what they pass, and thus make such a trip the more interesting.
At low water on the river front of the meadow that lies south of Rocky Neck, may be seen close to the water's edge a small ridge of rocks which the student should especially notice, as they are composed of the basic rock Porphyrite, and no other exposure of this rock is known in Hingham. Following the shore north of the porphyrite and just where the land rises from low and marshy ground, the first rocks which appear above the surface and rest-
48
History of Hingham.
ing somewhat back from the beach are conglomerates. Proceed- ing further a short distance, two dikes of diabase jut upon the beach, and not far inland may be seen to have cut through con- glomerate, the line of junction on a facing of one of them towards the water being distinctly perceptible The first of the dikes is about 450 feet from the porphyrite on the line of the beach, and the second about 40 feet further. The former of these will be more particularly mentioned before the close of these remarks upon Rocky Neck. Beyond the dikes, extending over the beach and along the shore for 350 teet or more, is a confused mixture of melaphyr with other rocks, petrosilex, porphyrite, granite, quartz- ite, etc. In portions the melaphyr forms with them a conglom- erate of which it is by far the larger part. Other portions can hardly be designated as conglomerate, being apparently the result of the intrusion of the melaphyr in a molten state among pebbles and masses unconsolidated, and absorbing them in its substance, each being now found surrounded entirely by the melaphyr.
It is in this portion of the rock of the shore that there is found much good red jasper, affording cabinet specimens of some beauty. The formation of this was clearly due to the chemical action arising from the union of the molten melaphyr with the material invaded. There are some veins of quartz found in the rock and others of an impure, buff-colored epidote.
Following this mixed melaphyr and conglomerate and less than 100 feet from it, is a very typical conglomerate containing peb- bles of granite, quartzite, and petrosilex. This extends about 90 feet. The jointing in this may be noticed as north and south.
About 80 feet from the conglomerate, melaphyr appears and extends for the considerable distance of about 500 feet. In it may be seen veins of quartz and also of the yellowish, opaque epidote mentioned above as occurring in the mixed melaphyr and conglomerate, but in far greater abundance. This melaphyr at its termination abuts directly against conglomerate, the line of demarcation being distinct and nearly vertical, though in places this does not clearly appear. There is undoubtedly a fault here. The conglomerate from the junction of the two rocks extends along the coast line about 240 feet. In this conglomerate is an east and west dike four to five feet wide. Melaphyr follows for some 50 feet or more, of a character similar to that before de- scribed as mixed with other material.
A bay in the land here occurs, and crossing it westerly on the beach at low tide the visitor finds cliffs of melaphyr which form a jutting point into the water. Crossing this a second bay is reached at a distance of about 100 feet. Here the rock displays the characteristic nodules that lead to its designation as amyg- daloid. Indeed a large portion of the melaphyr of Rocky Neck is finely amygdaloidal, and affords good specimens of this variety of the rock. On the beach here there is a protruding flat surface of rock, a yard or so in diameter, on which may be seen glacial
49
The Geology of Hingham.
striæ, though probably exposed there to the elements for centu- ries. These lines are northwest and southeast, and south 30° east. Other lines on a neighboring rock are northwest by west and southeast by east. On the westerly side of the bay granite appears in a high cliff towards and extending into the water.
By ascending this cliff, passing over it to its western declivity and descending to the narrow beach at its base, which should be done at low tide, a dike exposure may be seen of much interest. It is what is called a double dike, the molten material having made its way to the surface within two contiguous joints in the granite. The larger portion has a width of about eight feet, the
FIGURE No. 3.
smaller one about one foot, and they are separated by about one foot of the invaded rock. See Figure No. 3. This double dike slopes to the south from the vertical at an angle of 45°. This is the extreme western end, on Rocky Neck, of the dike first mentioned as appearing on the eastern shore. It does not present there VOL. I .- 4
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History of Hingham.
or generally over the surface of the neck its double character be- cause obscured by the soil. Across the water of the river, on Nantasket where it reappears, it shows itself double.
THE GLACIAL PERIOD.
A pretty full notice of the great glacier that rested over the North, and the phenomena attendant upon its advance and final melting away, has been given in the preliminary remarks. We have now only to treat particularly of the traces left upon the surface of the town by its passage. Those who have atten- tively read what has been expressed will understand that the decomposed material of early rock formations making up the soil of the territory of Hingham prior to the advent of the ice was largely borne away by its movement, the solid rock founda- tions being laid bare, whilst a large part of that which now forms the hills and covers the valleys was brought forward by the on- ward progress of the glacier from more northern localities. The whole of the earth thus disturbed and redistributed is known as Drift. Much of it was materially changed in the transportation. That directly beneath the glacier, and subjected to its enormous pressure and to great friction upon the rock surfaces below, was reduced to fragments, and even to the finest particles. The masses of rock, too, which were borne on beneath the glacier, that escaped destruction, were mostly smoothed, and often striated, like the rocky strata over which they passed. The part of the drift thus subjected to the crushing and grinding action of the glacier is known as Till. The definition of this term "Till," as given by James Geikie, the author of the exceedingly valuable work, " The Great Ice Period," is " a firm, tough, unstratified stony clay, with no very large bowlders, and having stones of a peculiar shape." The stones referred to are such as are oblong without being symmetrical in outline, and which exhibit striæ most often in the direction of the longest axis. Till constitutes the lowest member of the drift deposits. It is the " moraine profonde," or " ground moraine " of foreign geologists, the " bowlder clay " of most writers, the "hard pan" of our townsmen. It owes its compact and tough character undoubtedly to the immense pres- sure of the ice.
A considerable portion of the drift which was borne in the body of the glacial sheet itself, and thus escaped its grinding action, upon the final melting of the ice was spread loosely over the whole surface to a varying depth of from one to ten feet, and in some places to a much greater thickness. It is generally com- posed of gravel and sand with enclosed pebbles, and often contains an abundance of bowlders of large dimensions. Like the till, this upper drift is unstratified ; but neither the bowlders nor pebbles in it are striated, as is the case with part of those of the former.
51
The Geology of Hingham.
This is often called the Upper Till. It rests upon the general surface of New England, overlying the true till where the latter exists. It is easily distinguished from it by its somewhat dif- ferent composition, containing comparatively but little clay, and being much less compact, from not having been subjected to such great pressure. Its color, too, is generally yellowish, arising from the oxidation of the iron contained in it.
There is yet a third glacial deposit to be mentioned ; it is known as Modified Drift. This undoubtedly owed its origin generally to the action of rivers, which upon the melting of the ice-sheet swept over it and conveyed the rock masses, gravel, and sand, with which it was laden, to many localities where they are now found.
Having thus given an account of the origin of the drift deposits and their dissemination over the surface of the land, it remains for us to present the views of those who have made a special study of glacial phenomena respecting the peculiar hills that prevail in many sections over which the ice-sheet rested, and which form a predominant feature in the topography of the town ; and also of the less elevated summits and ridges known by geologists as Kames, which likewise present themselves prominently over a large part of its territory. The first of these, the peculiar hills referred to, are what have been called by the Irish geologists " Drumlins," a name of Irish derivation, signifying a long, rounded hill, - and by Professor Charles H. Hitchcock they have been called " Lenticu- lar Hills," from their lenslike form. We will first dwell upon these hills, upon the grooving and striation of the rocks over which the glacier advanced, and upon what are known as "pot-holes," as phenomena of the period under consideration ; postponing re- marks upon the later drift deposits and much other matter con- nected with the passing away of the ice, which will be presented when treating of the Champlain Period.
DRUMLINS, OR LENTICULAR HILLS.
These remarkable elevations are found in many towns of east- ern Massachusetts, but nowhere are seen to form more interesting features of the landscape than in Hingham. Baker's Hill, Otis Hill, Prospect Hill, Great Hill, Turkey Hill, and Pleasant Hill at Crow Point are all elevations of this character. They are com- posed, wherever found, mainly of the lowest member of the drift, the till, or bowlder clay, having generally but a thin deposit on their surface of the gravel and bowlders of the upper drift. They vary much in size, sometimes presenting themselves as mere hil- locks, but often found half a mile or more in length, and not infrequently over a mile. In form they are generally oval, more or less elongated, having symmetrical, rounded summits, with gen- tle slopes in the direction of their longest axes and much steeper ones laterally. In height they sometimes exceed two hundred
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