The great artisan incorporated three main features in the mosaic beneath
the New England upland, and from them erosion developed the major
pattern of the present landscape. The three units of the pattern
comprise a somewhat heterogeneous but durable foreground in the east, a
weak inlaid design in the center, and a moderately homogeneous and
durable background in the west. The foreground and background are simply
a suitable base for the younger, central feature of the design--an inlay
which was completed in Triassic time, while the mighty dinosaurs were
beginning to gain confidence as the new rulers of the earth. Skillful
artistry and complicated technique were expended on the Triassic inlay,
for in part it was rolled in, partly melted in, and some of it was cut
in amid the tougher materials now found on either side.
_The Red Rock Basin_
The youngest ingredients which were incorporated in the inlay are a
series of fine-grained red sandstones and consolidated clays or shales.
They are horizontal layers, turned up slightly at the edges of the
lowland, but elsewhere they lie in almost horizontal beds that extend
from South Hadley through Chicopee (Chicopee shale), Springfield, and
Longmeadow (Longmeadow sandstone) to a point just south of Hartford.
Near the hills which form the eastern boundary of the lowland these
fine-grained sediments locally give way to coarse tabular deposits of
angular gravel, which appear along the base of the Wilbraham Mountains
and again in Mount Toby and northward. The deposits are isolated or
detached masses which resemble fans emerging from mountains, not unlike
the more modern sands and gravels which the Westfield River left where
it emerged from the western hills. But the Triassic gravels are red, and
they are firmly cemented into conglomerate; yet it is plain that this
part of the inlay was made by washing and rolling the red muds, sands,
and cobbles into a depressed basin waiting to receive them.
The southern part of the basin was deepened, and the highlands were
rejuvenated spasmodically from Springfield to New Haven. The sinking of
the lowland on the west and the rising of the highlands on the east took
place along a fracture plane, commonly called "the eastern border
fault," near the eastern limits of the red sediments throughout that
part of the valley. The rocks composing the alluvial fans are flexed
sharply downward east of Portland, Connecticut, like compressed pages in
a book, where the great eastern mountain block pushed obliquely against
them. In this way the mountain range was renewed as erosion wore it
away, and the basin was deepened periodically as the wash from the
highlands filled it. The intermittent uplift sustained the growth of the
fans along the edge of the lowland, but the frequent recurrence of
movement never permitted these graded accumulations of waste to extend
far out from their mountain sources.
The great fracture, which sharply delimits rocks of different origins,
and the deformation in the strata near Portland record, as surely as the
writings of any human historian, a tale of periodic rock compression and
paroxysmal release that must have been accompanied by violent tremors.
Connecticut and Massachusetts had their earthquakes and had them as
violent as any now originating in the western ranges of the United
States and Mexico; but happily they shook a land which was overrun by
the dinosaurs, and which was not yet ready for human habitation.
[Illustration: Fig. 7. _Map of Mount Toby showing gorges filled with
conglomerate._]
Near the northern terminus of the Triassic basin the eastern boundary
was not subject to intermittent and violent movements during the later
stages of sedimentation, as it was in the south. Instead, the youngest
part of the red-rock inlay consists, in some places, of unfractured
boulder beds which were washed far out towards the center of the
lowland; elsewhere, landslides brought masses of rock debris upon soft
red and gray shales, which may have accumulated in shallow lakes; in
still other localities, long stringers of red sediment reach far back
into the eastern highlands. Many boulders in the conglomerate at the
south end of Mount Toby are eight feet in diameter, and torrential
mountain streams brought them to their resting place. A few are
scratched and grooved, much like the boulders in the till left by the
ice sheet; perhaps they signify the presence of snow fields and glaciers
in the mountain range, but the scratches may have been acquired by
avalanching. The landslide masses buried in the shales at the Sunderland
caves show that the mountain front was steep, and the ancient talus or
slide rock near the Central Vermont Railroad south of Roaring Brook
shows plainly that the mountain front was a precipitous cliff of
granite. The stringers of conglomerate extending eastward into the
granite upland south of Montague, north of Leverett station, at Amherst,
and again near Granby, are alluvial fill in ancient mountain gorges.
This old mountain mass stood out as a long, straight range extending
from a point east of New Haven northward into New Hampshire. It was of
moderate height in Connecticut, but it became higher and more rugged to
the north; glaciers may have nestled around its crest east of Deerfield,
and its front was an impressive slope of slide rock. Granite gorges with
tapering gravel plains, dry one day and raging torrents the next,
fingered eastward into the mountain block. At that time the Connecticut
Valley was much like the land east of the Sierra Nevada in California,
where greater contrasts in heights and depths are to be found than in
any other part of the United States.
_A Dinosaur Diary_
Like the valley east of the Sierras, the depression in central
Massachusetts contained playa lakes and intermittent streams. Sand
brought by the mountain torrents clogged the channels and spread into
broad alluvial plains, while silt accumulated in muddy lake basins.
Black sandy shales now mark the sites of the lake beds, and their black
color comes from the coaly remnants of Triassic plants. Some swampy lake
margins supported peat bogs, which have been preserved in coal seams two
to three inches thick between Granby and South Hadley. Many of the lakes
lasted long enough to become stocked with half a dozen species of fish.
But the fish led a precarious existence, and their skeletons were buried
in great numbers in the upper lacustrine layers when the lakes dried up,
and dust and sand drifted over the parched basins at Durham,
Connecticut, and at Sunderland, Massachusetts. The remains were interred
even more effectively when cloudbursts in the hills brought thick layers
of gravel out over the ancient lake beds.
Most of the lakes and ponds were ephemeral, but the fact that their
presence was more than a mirage in a Triassic desert is clear from the
ripple-marks retained on their sun-hardened surfaces, and from the
impressions of objects which touched them while they were still soft.
Stray series of parallel furrows record the passing of drifting shrubs,
and the abrupt disappearance of rain-drop imprints at a well defined
line in the hardened mud marks the exact position of the water level in
a few of these Triassic water bodies. Footprints register the activities
carried on by a bizarre animal population. Beside the road to the French
King Bridge and in the river bed at Turners Falls the ripple-marked
surfaces contain the impressions of many feet, and the dinosaur tracks
at "the Riffles" beside the Northampton-Holyoke highway are known
throughout the country. In Connecticut, Middletown and Durham are famed
for their tracks, and the impressions left in the playa beds by muddy
feet are so widely distributed throughout the lowland that it must have
taken a lot of walking by many generations of dinosaurs to leave such an
ample record.