The protracted story of central Massachusetts might be that of many
another section of eastern North America, except for minor details. In
Cambrian time an inland sea, well stocked with simple marine organisms,
washed the shores of an archipelago which extended north and south
through the Berkshire Hills, the Green Mountains, and the Notre Dame
Mountains. Composed of rocks which themselves had had a long and
involved geological past, the islands rose intermittently as streams and
waves wore them away. Clear water and sandy beaches stretched along
their western shore, and the original Adirondack Mountains were just
visible from the summits of the higher islands. Swift streams raced down
their eastern slopes, carrying gravels, sands, and silts into the
eastern arm of the sea, and only free-swimming animals could survive in
its turbid waters. For a time, volcanoes erupted and fumed along the
entire eastern coast from Thetford Mines, Quebec, to Plainfield,
Massachusetts, but their activity was short-lived. Only the streams
which drained the broad islands endured, and they never ceased to pour
mud into the eastern ocean. Gaps in the island chain permitted some of
the free-swimming organisms to migrate to the western sea, where
bottom-living plants and animals were actively secreting the limy shells
and skeletons which helped build thick deposits of Cambrian limestone.
These conditions continued into the ensuing Ordovician period of
geologic time, but gradually the situation changed. Again the volcanoes
renewed their activity, and masses of dark peridotite were intruded
along the eastern shore; the island chain rose rapidly, and the straits
closed. The elevated land began to expand outward, and folds spread
eastward on the east and westward on the west, like waves from a center
of disturbance. So great was the pressure that portions of the old land
were sheared outward over the folded sediments. The Taconic disturbance
was on from the city of Quebec to the city of Washington; and the
streams, like ants, kept at their endless task of carrying sand and
gravel into any and every depression they could find. They piled up
great thicknesses of Silurian sandstone in Maine and New York, and so
effectively did they tear down the Taconic Mountains that the Silurian
sea was ultimately able to penetrate the region from Thetford Mines,
Quebec, almost to White River Junction on the Connecticut River.
[Illustration: Fig. 13. _Block diagram showing main features of
central New England during middle Ordovician time._]
[Illustration: Fig. 14. _Block diagram showing main features of
central New England at the end of Ordovician time._]
[Illustration: Fig. 15. _Block diagram showing main features of
central New England during the Devonian period._]
One period later a Devonian sea followed in the wake of the Silurian
sea, but its waters penetrated even farther south to Leverett,
Massachusetts. The quartz gravels of its advancing beach covered the
worn flanks of the Taconic folds. Sea animals left their shells to form
a bed of limestone which may be seen today at Bernardston. But again the
sea was shouldered aside by the restive land, which rose from Gaspé to
Virginia. Much of the region affected by the Taconic disturbance was
elevated again, and a broad band of Devonian sediments was folded
closely through northern New Brunswick, southern Quebec, northern Maine,
northern and central New Hampshire, and central Massachusetts. Granites
welled up into the sediments, and dikes filled all the fissures. The
baking, stewing, and reinforcing they gave to the older sediments made
them so firm that they are still one of the most coherent and resistant
series of rocks in New England and maritime Canada. This was the
Shickshock or Acadian disturbance. Meanwhile the first forests took root
on the long piedmont plains that spread from the rising mountains
westward into the Catskill Plateau of New York State (Catskill
sandstone) and eastward to the coast of Maine (Perry formation).
The margins of the piedmont plain sank. Vast, luxuriant swamps succeeded
the old forests in Pennsylvania on the western piedmont, and in Rhode
Island, Massachusetts, and Acadia on the eastern piedmont. The swamp
vegetation later became the coal seams of eastern North America, and
well does this time merit its name--the Carboniferous period. The
Shickshock Mountains remained in the hinterland forming highlands from
Spencer, Massachusetts, westward into New York State; but they were
shorn of their crags, and only on rare occasions were the streams swift
enough to carry silt into the swamps and to bury the accumulated peat.
[Illustration: Fig. 16. _Block diagram showing the main features of
central New England during the Carboniferous period._]
[Illustration: Fig. 17. _Block diagram showing the main features of
central New England in early Triassic time._]
[Illustration: Fig. 18. _Block diagram showing the main features of
central New England during late Triassic time._]
Torn and twisted as New England had been by the two previous
disturbances, it was to suffer yet again. The entire northern section of
the eastern coal swamps began to rise, and the movement spread southward
through New Jersey, eastern Pennsylvania, Maryland, Virginia, the
Carolinas, and Georgia. Granites insinuated themselves once more into
fissures in the elevated landmass; the rocks were pushed outward from
the raised block; and the sediments of the coal fields were thrown into
folds which diminished in magnitude towards Ohio on one side and Cape
Breton Island on the other. This was the Appalachian Revolution. When it
was over, even the youngest sediments were interlaced with granite
sheets and dikes; they were cooked hard in hot spring waters; and they
were crumpled into close, long north-south folds. The landscape was
changed completely: mountains had replaced the peat swamps; and from
their summits alpine glaciers were plucking rock fragments which they
dumped into the Boston basin. Streams, too, cut deeply into the
mountainous upland, but there were no other local basins in which the
fluvial debris could come to rest.
This was, in brief, the course of events which transpired in that era of
geologic time called the Paleozoic. Twice as long as all ensuing time,
the era was one of kaleidoscopic change, with placid seas, eruptive
volcanoes, swift streams, and towering mountains competing for the lead
roles in three rather similar historical cycles. When the Paleozoic era
was over, the matrix of tough, resistant rocks was ready for the
delicate inlaid design which was imposed upon it in the Triassic period.
There was nothing tranquil about Triassic time. While hot springs, born
in the cooling granites, still oozed from rents in the mountainsides, a
tremendous 100-mile-long rift tore through the east margin of the old
Shickshock Mountain foundation. The rift was a clean break at some
places, but elsewhere it was splintered and offset. Each northern sector
of the break invariably ended west of the beginning of a southern one,
and the intervening rock is characterized by multiple fissures with more
or less displacement of their walls.
The block east of the rift moved south and rose, while that to the west
was depressed into a tilted and asymmetric basin. Mountain streams
flowing eastward to the Atlantic were caught at the base of the rift,
and a new set of torrents dashed down the west-facing scarp of the
elevated block. After every cloudburst these new streams left their
contributions of boulders in screes along the east side of the basin.
The gravels steadily increased in thickness, covering the hills and
valleys that furrowed the lowland floor. Much of the ancient relief
still lies buried beneath the fill, but some of the eminences were
exhumed one hundred and fifty million years later and have received
man-given names like Mount Warner and Bernardston Ridge. As the basin
subsided vertically for more than a mile, the mountain streams spread
fans westward across most of its floor, restricting the contributions of
the western rivers to a zone which is now less than two miles wide. The
largest of the eastern rivers wore a valley three miles wide where it
entered the lowland northeast of Granby.
Then volcanoes broke loose in the basin floor. Lava oozed through the
sand west of the Notch in the Holyoke Range, and it frothed out of the
openings or was blown violently from them. But by sheer persistence the
rivers still dominated the scene as volcanic activity waxed and waned,
and 1,500 feet of alluvial wash piled up around the volcanic cones. The
energy of the volcanoes was ultimately spent, but for some time lava
poured out of craters along a line extending southward from the main
eruptive center, and from a second center which approximates the course
of the Connecticut River from Sunderland to Turners Falls. It flowed
westward into the middle of the basin in a series of sheets until it was
400 feet deep; it pressed upward against the sand plains along the
western hills; it surged east up the fan slopes where it ended in a
frothy wall; and it spread southward from these two centers and from
others to New Haven. The lava, now tilted, gives substance to the
Greenfield Ridge, the Mount Holyoke and Mount Tom Ranges, and the long
line of hills that pass through Hartford and Meriden.
Spectacular was this outburst in its time, and profound was its
influence upon later scenery, but short was its duration. Before weather
could redden the lava surface, streams washed gravel over it; and only
at the main centers between the Mount Holyoke Hotel and the
Amherst-South Hadley road were the volcanoes able to hold out against
the relentless activity of running water.
The block east of the rift continued to move southward and to rise,
while the streams draining it entrenched themselves in an effort to
remain at grade with the basin floor. The moving mountain mass pushed
the lava flow up on end and twisted its eastern edge around, dragging it
along to the south. The rock splinters which were formed in the process
sliced the basin sediments into small blocks, some of which can be seen
north of Turners Falls and also at the Holyoke Range. Ultimately the
upward and southwestward movement along the rift piled the eastern
blocks against the more westerly ones, pushing the west side of each
eastern block up on the east side of the adjacent western one, and
depressing its eastern side more deeply into the basin floor. The many
fractures which were made weakened the basalt lava sheet along certain
zones where, in recent time, the elements have worn the notches in the
Holyoke Range.
[Illustration: Fig. 19. _Block diagram showing the main features of
central New England at the opening of the Cenozoic era._]
[Illustration: Fig. 20. _Block diagram showing the main features of
central New England at the present time._]
Streams from the eastern highland stubbornly filled up the holes and
planed off the raised blocks during the entire period of intermittent
movement. In the midst of the tussle between earth forces and fluvial
agents the volcanoes again broke into explosive eruptions, and volcanic
ash filled many of the block-like depressions all the way from Granby to
localities south of Holyoke. Then the fiery vents cooled, and the earth
movements diminished in their vigor. But they left a mountain front so
steep that talus and landslide deposits accumulated at its base near
Mount Toby; and the block mountain range was so high that glaciers may
have wreathed its summit. The mountain mass descended southward, and it
was penetrated by at least one low pass northeast of Granby.
In the basin itself, alluvial fans encroached from the eastern mountain
front, but out in the middle of the valley ephemeral playas and shifting
lakes were numerous. Rushes fringed the lake shores; fish stocked their
waters; and dinosaurs lumbered over the adjacent flats. The region was
one of violent rains and seasonal droughts, of hot days and frosty
nights--a semi-desert country lying in the lee of the Appalachian
ranges, somewhat as the intermontane valleys of the West lie in the rain
shadow of bordering mountains. Eight thousand feet of sediments poured
into the Triassic trough while these conditions lasted, but the
situation altered slowly as the Jurassic period dawned.
Throughout earth history, vulcanism and mountain-making have been
spasmodic events; but so long as rain has fallen and water has run
downhill to the sea, the unspectacular rivers have never relinquished
their task of reducing the lands to the lowest grade on which water will
flow. During all of the Jurassic and Cretaceous periods, and even into
the Eocene epoch of the Tertiary, New England's rivers worked towards
this end, and they came as close to attaining their goal as the restless
earth has ever permitted them to do. The region from the Atlantic to the
bases of the Green Mountains and the White Mountains was reduced to a
broad, faintly terraced erosional plain. Not all of it was leveled, for
Mount Wachusett, Mount Monadnock, the summits of Mount Greylock and
Mount Ascutney resisted the wear and tear of the weather and of running
water, and retained some of their original stature. At the headwaters of
the streams the Green Mountain chain and the White Mountains also
withstood reduction to the common level, forming the divide between St.
Lawrence and Atlantic drainage. Such rivers as the Merrimack, the West,
the Deerfield, and the Farmington followed somewhat different courses
than they do today, for some of the drainage heading in the Western
Upland of New England flowed straight across the red-rock valley to the
sea.
During Tertiary time the entire region rose vertically as a unit. The
rise was intermittent, punctuated by long stillstands of the upland with
respect to the sea. One of the earlier uplifts carried the land some 200
feet higher; and although the rivers maintained their courses, they
deepened their valleys and ultimately widened them into broad, open
plains far back towards their headwater reaches. In the resistant rocks
on either side of the red-rock basin the valleys were sharp and well
defined, but in the soft Triassic sediments the rivers cut wide swaths,
nearly eliminating the low divides which kept them in their independent
courses.
In Middle Tertiary time renewed uplifts occurred, and ultimately the
strathed surface was elevated 1,800 feet inland at the Green Mountain
divide. Once more the rivers started busily cutting down; but in a
protracted stillstand, while the New England upland still lacked 700
feet of its present elevation, the Atlantic Ocean planed off the hills
in southern Connecticut as far north as Middletown, and the Farmington
River adopted a more direct route across the marine plain to the sea.
Before the West, Deerfield, and Westfield Rivers could lower their
channels to grade in the reinforced rocks of the Eastern Upland, a
tributary of the Farmington worked headward along the poorly
consolidated red rocks of the basin and diverted the waters of the
northern streams into its own channel. This was the birth of the
Connecticut River, and in late Tertiary time, the energies of the
new-born stream were effectively expended widening the whole of the
Triassic basin. Even some of its larger tributaries developed wide
valley floors with steep walls in the hard crystalline rocks of the
uplands. Only the lava flows and the buried old-rock mountains withstood
planation in the red-rock basin. The flows form such trap ridges as
Greenfield Ridge, the Mount Holyoke Range, the Mount Tom Range, the
Hanging Hills of Meriden. Exhumed mountains are typified by Mount
Warner.
All of northeastern North America was raised to great heights in late
Pliocene time, and the Atlantic Ocean withdrew at least fifty miles
southeastward from the present shoreline. The rejuvenated rivers
deepened their valleys, forming narrow, sharply incised canyons like the
gorges of the Hudson and the Saguenay; and the Connecticut made a deep
groove in the lowland floor, cutting to depths which have been partly
disclosed by drilling at the Calvin Coolidge Memorial Bridge and the
Sunderland Bridge.
While the land stood in this high position, one winter's snow in the
White Mountains failed to melt before the next began to fall. Snowfall
accumulated upon snowfall, covering not only the White Mountains, but
all of Canada and New England; and the Ice Age was here to stay more or
less continuously for a million years. The ice piled up against the
highest mountains and ultimately rose so far above them that it slid
over their tops without attempting to detour around them. Its surface
may have been 13,000 feet above sea level in northern New Hampshire, and
its surface slope, which is estimated at 150 feet per mile, would give a
thickness of 10,000 feet at Northampton. The continent yielded slowly
under this great load, and it sank until all of the elevation gained in
the Pliocene movement was wiped out, and more besides. The ice radiated
from the centers of maximum accumulation--at first from the White
Mountains, and then from northern Ontario, and finally from Labrador.
The continental glacier crept southward to Long Island and Martha's
Vineyard, where its front melted in the waters of the Atlantic as fast
as new ice came up behind. It dragged and pushed and carried debris,
only to dump it in a hummocky ridge, like a rampart, to mark its
farthest advance.
At last the glaciers started to melt even faster than new masses moved
down from the north, and the ice front began to recede 400 to 700 feet
per year. The sea followed it, up the Hudson, up the St. Lawrence, in
over the coastal lowlands for a short distance; and everywhere pounding
waves made beaches at the water line. And in the path of its slow,
deliberate retreat, the glacier left rock debris--boulders on the hills
and in the valleys, boulders everywhere; all the landscape was marred
and desolate.
The ice had weighed the pre-glacial valleys down more deeply in the
north than in the south. One such valley was the Connecticut Lowland, in
which water gathered to overflow-height at Middletown. Thus Lake
Springfield came into being, and it spread northward as the ice front
receded. North of the Holyoke Range another lake formed, and this
northern body of water has been named Lake Hadley. Streams flowed off
the ice, off the hills--flowed with unimpeded vigor, for there were no
trees or grass to retard the run-off. Deltas grew out from the shores,
and annual layers of clay settled on the lake bed.
The ice grew thinner, its area smaller, and its load lighter; and as
Mother Earth lost her heavy burden, the land rose, more in the north
than in the south. The differential rise decanted the water southward
out of the lake basins, and the seas retired from the coastal lowlands.
Old shores and sea beaches remained as flat terraces sloping gently
southward. The rivers raced down the steep beach slopes to the old lake
floors and sea bottom. They cut their channels deeply into the
unconsolidated deltas and meandered back and forth over the flat,
ungraded lacustrine plains, as if uncertain where to flow. They flooded
the lands in the spring, leaving loose sand and silt for the winds to
blow when the water was low. Sand dunes rose near the river banks at
North Hadley, Sunderland, Hatfield, and South Deerfield; but the march
of the dunes was arrested as post-glacial vegetation repossessed the
land. It was at this point in the story that man found and settled the
Connecticut Valley, becoming a witness to the geologic work of the river
and an aid to the work of the wind as his plow bared the fertile soil to
the elements.