[Illustration: a. _A dinosaur walked from the raindrop marked
surface at the right to a shallow pond at the left._]
[Illustration: b. _Volcanoes ejected much ash and many bombs to form
the Granby tuff._]
Some of these three-toed animals were like the modern lizards and walked
on all four feet; but the great majority walked on two feet and, like
the kangaroos, used their tails to balance their bodies, and their short
fore limbs to support them when they crouched. In any single playa
deposit, variations in the sizes and kinds of footprints reveal that
many individuals made them; yet strangely, most of the tracks at any one
place are headed in a single direction. Apparently the herd instinct
must have been strong in these reptiles, as it is in kangaroos or in a
flock of turkeys, all following a leader, with only an occasional
individual going off to one side or back-tracking in a display of
independence. And so the dinosaurs dominated the life in the early
Connecticut basin, as it sank and trembled, and as mountains rose to the
east; on dry days and days of cloudburst, on hot days and days when
frost crystals formed in the mud, they roamed the plain, as the lowland
settled nearly two miles and filled to the brim with red sands, muds,
and marginal gravels.
_Volcanoes_
Red is the predominant color in the central inlay of the New England
design, but greens and blue-black lines have been worked into the
pattern. The dinosaur-ridden basin has a rim south of Middletown in
Connecticut, and another north of Holyoke in Massachusetts; it lies just
west of the dinosaur-track ledge near Holyoke, and the tracks themselves
are only thirty feet above the bottom of the basin. The rim is an odd
ensemble--now red and now green; here solid and hard and black, there
soft and fragmental and crumbly. The fragments may be angular or round;
sandy or glassy; dense and solid, or full of bubble holes like molasses
taffy. The whole looks like the spread-out ash dump from a giant power
plant. And not only does it resemble an ash heap--it _is_ the ash heap
of a volcano; and the hard black layers within it are lava flows
interspersed with the heavy falls of ash.
[Illustration: Fig. 8. _Map showing agglomerate burying a fault
scarp on the power line through the east gap of the Notch._]
The ancient ash heap grows thicker east of the Connecticut River, and it
is more than 3,000 feet from top to bottom around a series of massive
blue-black rock-columns southeast of the Mount Holyoke Hotel. These are
the lava-filled necks of craters which became quiescent with the dawn of
the dinosaur days. The ash deposit, called the Granby tuff, grows
thinner eastward away from the craters and disappears completely
northeast of Granby, where a stream deploying from a valley in the
eastern mountains washed it away as fast as it fell and left coarse
gravel in the form of a huge fan.
The floor on which the ash came to rest was not everywhere the same.
Where now it crosses the Northampton-Holyoke highway and the
Amherst-South Hadley road it was a lava flow; but north of Granby and at
numerous places between the Hockanum and Amherst-South Hadley roads the
ash lies on conglomerate. Along the Amherst-Springfield power line, a
block of the conglomerate floor was pushed up five hundred feet above
the same beds farther west, forming a small block mountain which was
entirely buried beneath the ash. Similar block mountains can be observed
under the blanket of ash, especially on the south side of the Holyoke
Range; and renewed movement subsequently affected many of the blocks
north of Granby, where the ash deposit and even some of the sediments
laid down in the earlier days of the dinosaurs were fractured and
displaced. As a rule, along any one fault, the block on the east was
pushed up and moved southward; and the block on the west was pressed
down: as a group, the fractures may form the beginning of the great
eastern border fault which bounds the basin farther south.
The volcanoes which made the Granby tuff or ash bed erupted
intermittently for a long period of time. Usually, the river which
emerged from the eastern mountain range brought so much fluvial debris
that ash is not in evidence except in the immediate vicinity of the
craters located between the Notch and the summit of Mount Holyoke. Even
though alluvial sands and gravels supplant the tuff here and there, the
river did not succeed in closing or quenching those fiery vents. The
rocks now present recount a struggle in which, at times, the river
encroached upon the cinder cones; at others, the ashes choked the stream
and buried its alluvial wash.
While the volcanoes rumbled and erupted, earth forces intermittently
thrust the eastern mountain range southward and upward, dragging the
eastern margin of the lowland with it and upturning the sedimentary
fill, much as a plow might upend a layer of snow at the roadside before
shearing it off and pushing it out of the way. The relentless movement
caused the entire eastern floor of the basin to be broken into blocks;
the easterly ones were piled against the westerly, and their eastern
edges were pushed down into the basin floor and the western borders rode
up on their neighbors. Through all this tremendous disturbance the great
stream pouring out of the mountain pass kept the elevated blocks cut
down and the small basins filled in. Earthquakes, erupting volcanoes,
and shifting rivers made life for the dinosaurs troubled and a bit
uncertain.
Only once did the volcanoes dominate the situation in the valley, and
that was very early in their history. A group of vents, localized along
a southward trending zone about a mile west of the Notch, and another
group along the present course of the Connecticut River from Turners
Falls to Sunderland poured out billions of cubic feet of black basaltic
lava into the center of the lowland. Eruptions followed in such rapid
succession that the rivers never scoured the surface of the earlier
flows. Lava piled up 400 feet thick in the center of the basin east of
the Mount Tom Range; it moved eastward in a flow which thinned against
the fans of rivers issuing from the eastern mountain, and it ended in a
formidable wall of scoria confronting the mountain streams. Lava buried
the northern basin from Sunderland to Turners Falls and beyond, while
the southern basin filled from Northampton to New Haven. But lava
dominance was short-lived, and even before its bubbly surface reddened
to the weather, streams had covered it with gravel.
The lava flows are the most resistant materials used in the lowland
design. They form the ridge east of Greenfield in the northern basin.
The Holyoke and Mount Tom Ranges are remnants of these flows, tilted at
moderate but varying angles by the recurrent movements which enlivened
the epoch of dinosaurs and volcanoes.
[Illustration: Fig. 9. _Block diagram showing the main features of
central Massachusetts during volcanic stage._]
[Illustration: Fig. 10. _Block diagram showing the Triassic basins
of central Massachusetts._]
The most spectacular episode of lava extrusion was localized in a small
volcanic center situated about one mile west of the Notch in the Holyoke
Range. All flows in the range moved away from this center, and before
the great outpouring took place, minor explosive outbursts had built
cones of ash with bases up to a mile in diameter. Small lava tongues are
interspersed with the ash beds, and mixtures of sand and lava tell of
breaks through the 1,500 feet of sandy fill which was rapidly
accumulating in the basin. Throughout this early period of volcanic
activity the streams brought out so much wash from the eastern mountains
that they soon dominated the scene in Massachusetts, and in Connecticut
volcanoes gained ascendancy for one brief moment of geologic time, when
an early flow covered much of the valley from Hartford south.
_The Original Valley_
The first and oldest ingredients in the central design are entirely red.
The materials are fragments of older rocks--granite and gneiss, schist
and pegmatite, feldspar and quartz. They are invariably coarse, for
every layer of inwashed sediment has pieces over an eighth of an inch in
diameter, and only the coarser particles were smoothed. The finer
particles were not moved about enough to have their sharp corners worn
away. The pebbles and clay in the thick layers of conglomerate at the
French King Bridge were dropped by rushing, overloaded torrents
deploying on a lowland--a situation not unlike the one at Townshend,
Vermont, during the hurricane deluge. Only fine debris was transported
across the fans to the far side of the basin. The western hills made
small contributions of sediment; but their streams brought particles
which never exceeded an inch in diameter, and in quantities so moderate
that the fragments underwent some sorting and sizing as they were spread
over the lowland. From the very start the valley was deeper near the
east wall than the west; and the eastern mountain block was greatly
elevated, whereas the western block was simply a hilly upland.
[Illustration: Fig. 11. _Map of the old volcanic region near Mount
Hitchcock and west of the Notch._]
The edge of the eastern mountain mass is located at the French King
Bridge and passes half a mile west of Montague. Its location beneath the
younger fill is known less perfectly farther south, but it seems to
extend through Amherst, certainly west of South Amherst and Granby and
probably east of the Notch. At least two mountains rose above the
ancient lowland floor; the northern one is a long ridge of schist
between Bernardston and Mount Hermon, and the other is Mount Warner.
Mount Warner is an island of highland rocks in a sea of red sandstone
fill. The Bernardston ridge resembles a peninsula in somewhat analogous
sedimentary surroundings. The two eminences reveal the form of the
valley floor and the western hills at the dawn of the Triassic period,
for they were spared from destruction by burial, until deep erosion
exposed them again in Miocene time.
_Hot Springs in Central Massachusetts_
Hot springs the world over register their presence by leaving deposits
of unusual minerals, and they have left this sort of record at
Loudville. Here the coarse sandstones of the lowland rest upon gneiss,
and at the south end of the Loudville lead vein barium sulphate
crystals, called barite, formed in the sand before it was cemented into
solid rock. The crystals are the product of highly charged mineral
water, rising through the sands from a subjacent fissure. The fissure
itself is also filled with barite, and with galena and quartz as well.
It is the vein which was worked in the old Loudville lead mine. There
are other veins in the western and eastern highlands at Hatfield, at the
Northampton reservoir near Whately, and at Leverett. All are in
fractures which were still partly open when the valley first took form.
_The Marginal Uplands_
The rocks which formed the high eastern mountain range of Triassic time
and the rocks which made the old western hills and underlay the basin
floor comprise essentially a single group characterized by its
complexity. At one place the rock resembles sandstone, but the layers
stand on end; at another, it looks like shale, but the stone breaks
across the color banding instead of parallel to it; and at a third place
a fissure seems to have opened and had a crystallizing melt poured into
it. These tabular, filled fissures can be found nearly everywhere,
coursing in every direction and at all conceivable inclinations to form
a network that binds the older rocks into a firmly knit whole. The
fillings, or dikes, are like reinforcing rods, holding the rocks
together and withstanding the agents of destruction. Thus, the story of
the highlands has three distinctive phases,--a relatively young phase
when the interlacing reinforcements were poured into fractures; a
somewhat more remote stage, when the bedded rocks were crumpled into
their inclined positions; and an earlier stage, when the bedded rocks
were deposited. The geologic dates of these three events may vary from
one locality to another, and they certainly are different in the Eastern
Upland as compared with the Western Upland; but the events always
occurred in this sequence and constitute the broader aspects of the
story at all places.
[Illustration: Fig. 12. _Block diagram showing topography during
formation of the lead veins._]
_The Eastern Upland_
The Eastern Upland includes the land between the Connecticut Valley and
the Atlantic Ocean. At present, it has the general form of a broad
rolling highland with ridges and valleys that have a north-south trend.
Close inspection shows that the rocks in the ridges are different from
those in the long valleys. Also the layering of the materials ranges
from a vertical attitude, as at Ware and Brimfield, to undulating and
almost horizontal positions, as at Spencer and Worcester.
Through vertical and horizontal beds alike run those reinforcing
sheets--some tabular and vertical, called dikes; others also tabular but
horizontal, called sills; and some are just huge, irregular masses
without visible bottoms, called stocks and batholiths. Some of them,
composed of uniform, small, light-colored minerals, are granite; others
are made entirely of large minerals over an inch across and are called
pegmatite; a few, with cuneiform intergrowths of a dark mineral in a
light one, arranged like Arabic writing, are known as graphic granite.