Rocks record three distinct methods which nature employs in the
aggregation of minerals. The sedimentary rocks register the work of
wind, water and ice. Deposits left by wind and water are generally
stratified or bedded, and they, together with glacial deposits, are
composed of fragments which touch one another and are cemented at the
points of contact. Igneous rocks record the solidification of hot
liquids which injected themselves into older rocks or filled crevices,
and which, upon cooling, formed masses of closely fitting crystals. The
third group includes types which are crystalline like the igneous rocks,
and which may be laminated somewhat like the sediments; they show
effects of heating and squeezing until their original forms and even
their minerals were changed. These are the metamorphic rocks.
Anyone who wants an orderly record of geologic history will arrange his
rocks into these three groups--the sedimentary, the igneous, and the
metamorphic. In the Connecticut Valley the metamorphic rocks reveal the
ancient phases of earth history, and the sediments contain the details
of younger or later geological episodes. The igneous rocks have a wider
historical range; and, like the other types, they record a long period
of violence and upheaval which seems out of harmony with the placid
countryside for which they now provide a solid foundation.
_The Sedimentary Rocks_
The sedimentary rocks are built from the disintegrated wreckage of older
ones. The products of rock decay are picked up and dragged, or carried
in suspension or solution, by wind, running water, or moving ice. They
are deposited when and where the transporting agent can no longer
function. Such rocks are usually layered because the transporting power
of the carrying agent fluctuates. Bands of one kind of material,
separated by dissimilar materials above and below, are called beds.
The bedded or stratified rocks of the Connecticut Valley vary greatly,
from the coarse bouldery deposits in Mount Toby to the fine-textured,
red and black laminated beds at Whittemore's Ferry. Conglomerate,
arkose, graywacke, shale and even limestone are represented, but there
is little true sandstone. Sandstone is an even-textured, granular rock,
most commonly composed of cemented quartz grains. Its uniformity of
grain-size and composition reflects prolonged weathering of the original
rock and good sorting of the fragments as they were transported to their
new resting place. The sequence of exposure, transportation and
deposition was too rapid in the ancient Connecticut Valley to permit
appreciable decay and sorting; hence sandstones are absent. Limestones
and salt beds are likewise rare, but the metamorphosed limestones which
are found in the western highlands and in the Berkshire valley
demonstrate that limestone-forming processes played a significant, if
intermittent, part in the history of the region.
CONGLOMERATE is consolidated gravel. Pebbles and boulders of all sizes
are packed together by the stream which was moving them, and the
spaces between the larger fragments are filled with the sand that
settled in from the stream bed. The entire mass is cemented by silica,
limonite, carbonates or some other substance deposited by percolating
ground-water. The Devil's football near the Mount Holyoke House is a
famous piece which was dislodged from the hillside above; and
excellent specimens may be collected on Mount Toby, on Mount
Sugarloaf, and in the cut at Mount Tom Junction.
ARKOSE resembles conglomerate, but the individual grains consist of
mineral fragments, among which reddish feldspar is prominent. Quartz
and mica may be present, too; and all the pieces are
characteristically angular, commonly ranging from ^1/_1_6 to 1/8 inch
in size. The rock is red and crumbles easily. Beds of arkose alternate
with conglomerate on the steep sides of Mount Sugarloaf.
GRAYWACKE is light to dark gray in color, and the fragments composing
it are sand size pieces of older rocks. A few mineral grains, such as
quartz, may be present, but mica is rare. Graywacke occurs interbedded
with arkose in some parts of the valley.
SHALE is a thinly laminated sediment composed of microscopic quartz,
feldspar, mica and kaolinite grains. Most shales in the Connecticut
valley were deposited as muds in old lake beds. Some are red and
record ephemeral pools, but others show from their black color, their
coal layers, and their fish skeletons, that the water bodies in which
they accumulated remained in existence for a comparatively long time.
LIMESTONE is a rock composed of calcium carbonate, and it consists
essentially of an aggregate of calcite crystals or calcite fragments.
It will give off gas bubbles in a very dilute solution of hydrochloric
acid, and it exhibits other properties peculiar to the mineral
calcite. A thin, sandy limestone bed has been identified in several
sections of Holyoke.
_The Igneous Rocks_
Igneous rocks were once molten, and in this hot fluid state some were
extruded at the surface as lava flows. Congealed flows reveal the
motion, which brought them to their present resting places, in the
banding and streaks that are so evident in the patterns of steam holes
and minerals; but their massive structure bears witness to stagnation as
they hardened. Other molten masses insinuated themselves into
underground openings, where they solidified as intrusives, varying in
size from small dikes less than an inch wide, to huge masses that can be
measured in miles in any direction. Most of the igneous rocks in the
highlands of western and central Massachusetts are massive intrusive
types; light-colored varieties predominate, but some dark-colored dikes
cut the older rocks both east and west of the valley. Dark-colored,
massive and banded lavas are conspicuous in the ranges within the
valley.
Igneous rocks may be divided into three general groups for practical
classification, and each major group may be further subdivided. Rather
conveniently each of the major groups may be recognized by the prevalent
color of its rocks--whether dark, medium-colored, or light. And within
each major classification there may be flows, characterized by banded
structures and fine textures; small intrusives composed of well formed
crystals in a fine-grained groundmass; and large intrusives consisting
of goodsized, equi-granular crystals. Not all of these types can be
found in central Massachusetts, but the variety of igneous rocks is
surprising and offers some excellent possibilities for the collector.
_The Dark Rocks_
The dark rocks owe their color to iron-bearing minerals like olivine,
pyroxene (augite), amphibole and biotite. All of these minerals weather
to a rusty red surface, which is typical of their outcrops at many
places.
BASALT is a black rock, dense in some places but perforated with
bubble holes or vesicles, at others. It occurs throughout the length
of the Holyoke, Tom and Pocumtuck Ranges; and fragments of basalt are
abundant in the Granby tuff and agglomerate.
DIABASE resembles basalt but is distinguished by the thin, short
crystals embedded in it. These crystals of labradorite feldspar
resemble pieces of clipped thread, and they sparkle in reflected
light. Almost all dark-colored dikes and the slowly cooled central
portions of thick lava flows consist of diabase.
PERIDOTITE is a dark green, coarse, granular rock composed of olivine
and subordinate amounts of pyroxene. It occurs near Westfield and
Blandford, and at many places in Vermont.
_The Medium-Colored Rocks_
The medium-colored rocks contain approximately the same proportions of
light- and dark-colored minerals. The dark iron-bearing minerals are
relatively stable, but the light-gray feldspars decompose to kaolin and
give the weathered rock a chalky white surface. Surface flows of this
group are unknown in central Massachusetts, but the coarsely granular
intrusives are well represented.
GRANODIORITE PORPHYRY is a greenish-gray rock occurring in many dikes
in the western highlands. It has rectangular crystals of andesine
feldspar up to 1/8 inch across, and these have a dull porcellaneous
luster. A few dark-green amphibole crystals are only slightly smaller.
Both feldspars and amphiboles are embedded in a very fine-textured,
pale greenish groundmass.
GRANODIORITE is a gray equigranular rock containing flesh-colored
microcline feldspar, white andesine feldspar, greenish flakes of
chlorite, needles of amphibole and sparse grains of brown biotite. All
crystals are about ^1/_3_2 inch thick and commonly display a parallel
arrangement. This rock forms huge irregular masses at Williamsburg,
Whately and Belchertown.
_The Light-Colored Rocks_
The light-colored rocks are well represented by dikes and large masses
but not by recognizable surface flows in central Massachusetts. Their
exposures have rarely weathered much, because the predominant minerals
are quartz, microcline, orthoclase and albite, which resist decay.
QUARTZ PORPHYRY is a light gray rock that is found in dikes. It has
porcelain-white cleavable feldspars up to 1/8 inch thick, and dark
glassy quartz of equal size in a granular mass of very fine-grained
crystals. Intrusives of this type are numerous in the vicinity of
Whately.
GRAY ALBITE GRANITE occurs in many dikes and small irregular masses
throughout the highlands. All crystals have approximately the same
size and rarely exceed ^1/_3_2 inch in thickness. They consist of
white orthoclase and albite, dark sugary quartz, and brown to black
biotite.
RED MICROCLINE GRANITE is found in very large, irregular intrusives in
the highlands. The crystals are ^1/_1_6 inch or more in thickness. The
red color is due to the flesh-colored microcline. Quartz is dark and
glassy, and muscovite is the typical mica.
_The Metamorphic Rocks_
Metamorphic rocks were once sedimentary or igneous rocks which have been
changed by intense pressure, by heat, or by solutions moving through
them. Pressure usually produces a sheeted or foliated structure along
which the rock exhibits a tendency to part--somewhat like the pages in a
book that was bound before the ink was dry. Percolating solutions may
produce chemical alterations in the original materials and even
crystallize new substances along the foliated surfaces within the rock,
much as water circulating through cooled soil may solidify to ice and
cause heaving. Many of the rocks in the highlands bordering the
Connecticut Valley are highly foliated or banded in consequence of the
mechanical deformation they suffered when the ancient upland mountain
system was created. They include the slates, schists and gneisses. A few
massive types, like marble, serpentine and soapstone, owe their origins
chiefly to the effects of heat or of the hot, chemically charged
solutions which permeated them.
SLATES are fine-grained rocks characterized by flat, parallel cleavage
surfaces which usually cross the original sedimentary structure. They
were formed from shales, by shearing and compression during ancient
mountain-making movements. Slates crop out beside the station platform
at Brattleboro, Vermont, and at many places southward along Federal
Highway 5 to Greenfield.
SCHIST is foliated, too, but it is composed largely of cleavable
minerals, such as chlorite, muscovite, biotite and amphibole, which
are distributed along the cleavage surfaces. These minerals result
from the chemical activity of hot solutions circulating along a slaty
cleavage, re-crystallizing old materials, and bringing in new to make
these coarse mineral flakes. The schist receives its specific name
(biotite schist, chlorite schist, etc.) from the mineral which
accentuates its cleavage structure.
A few schists contain large crystals which bulge the schistose
surfaces outward around them. Garnet is characteristic in this role,
and a muscovite schist with garnets in it is called a GARNETIFEROUS
(or garnet-bearing) MUSCOVITE SCHIST. Other minerals with occurrences
similar to the garnet are microcline, albite, staurolite, amphibole,
tourmaline, pyrite and magnetite.
GNEISS is a banded rock containing cleavable minerals, but it lacks
the cleavage structure of schist. The cleavable minerals (biotite,
muscovite, amphibole, etc.) may give the gneiss its specific name, but
as often as not, the name is derived from the whole mineral
assemblage, or from an assumed origin, as in the case of granite
gneiss. As in the igneous rocks, the mineral ensemble is held together
by interlocked quartz and feldspar grains. Black-banded biotite gneiss
and hornblende gneiss are the most abundant varieties in the
neighborhood of the metropolitan reservoir east of Pelham.
MARBLE is a granular rock composed of calcite crystals. It is formed
when heat volatilizes the bituminous coloring agents of ordinary
limestone and simultaneously causes enlargement of the calcite grains.
It is the principal rock in the Berkshire Valley in which North Adams,
Adams and Pittsfield are located.
OPHICALCITE is a lime-silicate rock. It is formed by the chemical
reactions of hot solutions on limestone or marble at considerable
depth within the earth. The original calcite is converted into
diopside, garnet, vesuvianite and tremolite, forming a rock that may
be massive, or which may preserve some of the original bedded
structure. It is found in association with the crystalline limestone
and magnetite at the old iron mine, located one mile north of
Bernardston.
SERPENTINE is a dark-green rock made almost exclusively of the mineral
serpentine. It results from the reaction of hot solutions on olivine
and pyroxene rocks (peridotites). Serpentinite is present in the
Westfield marble quarry and at Zoar on the south side of the Deerfield
River.
SOAPSTONE is composed principally of talc. It, too, results from the
chemical activity of hot solutions ascending through serpentine and
causing the mineral transformation. Bodies of this material are
associated with the serpentinite at Westfield and Zoar, and northward
in sections of Vermont. It is mined for talc, but in colonial days it
found many uses. The colonists used cross-cut saws to make blocks for
foot warmers in their sleighs, to control the heat in the old
wood-fired ovens and to make water pipes before iron and lead were
available in adequate quantities. Many soapstone articles may be
seen--and purchased--in Wiggins Country Store and in other good
antique shops through the valley. One of the most primitive Indian
cultures in this region utilized soapstone pots, and exhibits are on
display at both the Springfield Museum of Natural History and the
Amherst College Museum.