Notes on Geology (1887)

Notes on the Geology of Bedford. [*1]

—————

In treating of the geology of Bedford we will make a few remarks on that most important feature to the farmer, the superficial deposits or subsoils. These may be divided into three kinds.

There is first the dark peat of the swamps and meadows that line the courses of the streams, and that along the Concord and upper Shawsheen have so great extent. They are not often of great depth.

Thence comes the Thanksgiving cranberry, and the meadow hay more fragrant than valuable.

In contrast to this soil of the lowlands is that which clothes the uplands, rounds the hills, and hides the underlying rock save where here and there a little ledge crops out. This is composed of a mass of sand, pebbles, and boulders with a good deal of clay. It is known as the boulder clay, and is supposed to have been brought under the ice of glaciers, as the irregular mixing of fine and coarse pieces of rock in a relatively unrounded state, its presence all over the hills, and a comparison with deposits under the glaciers of Switzerland and Alaska show us.

The ice came from the north and also rounded and scratched the underlying rocks, — like a dull plane, — and bore fragments of them to the south without much regard for up and down hill. Examples of such rounded and scratched outcrops are to be seen along the Billerica road, and the old Lowell turnpike and elsewhere. The direction of the scratches is from the north. A full record of the direction and locality of such scratches would be of scientific interest.

This boulder-clay soil is that of the typical old New England pasture. It is the home of the huckleberry, it supplies the stone for stone walls, but it is hard work to make it better or worse than it is by nature.

Between these two first kinds, or at least never under the boulder clay, come beds of sand devoid of clay, with few and rounded pebbles, with lines of successive deposit by water generally visible to the practised eye. [^1] These are well marked in the sand-pit to the north of the Shawsheen cemetery.

These sands lie partly under the swamps or near the present level of the streams, and may have been deposited by the brooks and rivers some time ago, but under the present condition of things.

In part, however, they are many feet above the present stream level, and are referred to as the work of streams while the ice was disappearing from the country. These high, level sand-beds are a favorite place for cemeteries, — being easy to dig in, — and pine trees, and are often opened for sand-pits. They are extensive near the south school house. [*2] A well marked one lies between Indian Hill and the old turnpike, and others are near the Concord river. [*3]

The soil is light and dry, needs fertilizers, but rewards their use.

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Underneath all the soil is — so far as we can judge from the outcrops — gneiss: that is, a mixture of quartz and feldspar with mica or hornblende so arranged that it splits easier in one direction than in any other. [^2] A split in this direction shows more mica than one in any other. In Bedford the strike of the gneiss is N.E. and the dip nearly perpendicular.

According to Hitchcock‘s map, in the S.E. corner of the township hornblende schisti.e. hornblende gneiss minus the quartz, and with less feldspar and mica, — is associated with the common gneiss. [^3][*4] This seems fairly accurate. Near the Lincoln line in connection with the yellow ochre, (216 Hitchcock’s state collection) comes a rusty mica schist. [??]

Just over the Concord line a diorite, a granitic variety of hornblende rock, has been opened up for railway culverts and for gold! Here also come seams of almost pure iron mica.

Throughout the eastern part of the township on both sides the Shawsheen, boulders of hornblende rocks are not seldom seen, but never predominant.

Some are of typical hornblende schist, others have a more massive appearance, some even remind one of a kind of diorite known as cortlandtite.

I have also found two small boulders of trap.

All these latter rocks are probably only thin seams or intrusions in the gneiss.

According to the map a little syenite should occur in the south-east corner. I have not seen it.

The gneiss is further riddled with veins of pegmatite. A good example occurs on the road that runs along the left bank of the Shawsheen. Pegmatite is, roughly speaking, composed of the same minerals as gneiss, in larger and more perfect crystals, without any special direction of splitting. Boulders of it are frequent.

Bedford is mentioned by Dana and Hitchcock as a locality for garnet. This mineral occurs in two ways. As reddish patches it comes in mica schist, up to an inch or two long, so full of enclosures that it seems to have grown around the other elements of the rock, and is still sometimes only one large crystal, recognized as such by catching the light from a face. [^4]

In the pegmatite they occur as sharply defined crystals of a deep red color. From a boulder on Mr. Rodman’s place I have seen them about a half an inch in diameter, and got them out in the nearly perfect shape of ikositetrahedron (211). [*4][^5][??] On the north slope of Indian Hill they also occur, and are liable to be found anywhere.

I know of no other mineral occurring in form worthy of remark, nor of fossils.

The glittering yellow specks that often can be seen on a fresh rock surface, and that in one outcrop of gneiss along the railway to Billerica, seem almost to replace the mica, are generally sulphides of iron, etc., and of no value. Gold is malleable, they are brittle.

If, however, their decomposition gives rise to the iron and sulphur springs of Bedford, I must take back my remark as to their worth.


SOURCE TEXT

  • [Alfred C. Lane’s] “Notes on the geology of Bedford” (1887) [ no scan ]
    in The Bedford book [1] (unpaged)

EMENDATIONS

  1. boulder clay, ∨ boulder-clay,
  2. gneiss: that is, ∨ gneiss, that is
  3. mica, — ∨ mica,
  4. up to ∨ up
  5. ikositetrahedron ∨ ikositetraedron

WORKS CITED


ANNOTATIONS

  1. This contribution was published without attribution.
    Brown gives Alfred C. Lane as its author. (BHB) p 45
  2. “the south school house”: the second South school
    Stood at the intersection of Forest Street and South Road. (CAM) p 46
    This site has since been consumed by Hanscom Field. (HPN) p 409
  3. Indian Hill was situated in the northeast of Bedford. (CAM) p 46
  4. cf. [ evidently ] Hitchcock’s “Geological map of Massachusetts” (1832) [ no scan ]
  5. “Mr. Rodman’s place”: 97 North Road (HPN) p 260
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