Part 3

Broad, open valley flats or straths surmount the steep V-shaped notches of both the Deerfield and Westfield Rivers. Surely, everyone who has paused at the lookout on the east summit of the Mohawk Trail has seen the upland sloping gently towards the Deerfield and then breaking sharply at the top of the present canyon. The same view confronts the motorist who drives from Adams to Cummington, just after he leaves the village of Plainfield. Here the shallow bowl in front of him holds no hint of the deep notch in which the Westfield flows. The gentle contour of the land suggests only the slow but methodical sort of change which comes with maturity. Those who favor air travel will see, as they fly over Mount Tom, a similar but more dissected strath reaching into the hills northwestward from Northampton. Aeroplanes flying the Boston-New York route pass over straths which have been trenched by the Connecticut along its course from Middletown to New London.

The straths are part of a mature, but ancient drainage system, which was graded a thousand feet above the level of the present streams and only a few hundred feet below the main upland. Certain broad depressions through the highlands east of the Connecticut Lowland suggest that this drainage pursued a southeastward course to the Atlantic, and that the river did not establish its modern course until the straths were elevated and notched.

The land level above the strath-margins is a still older surface from which the rock-benches were cut. The higher surface stretches to the horizon at Pelham, but Mount Monadnock and Wachusett stand conspicuously above it. And on the Mohawk Trail one must ascend the tower at the eastern summit before any higher land comes into view. Greylock's summit and the long chain of the Green Mountains attain greater elevations. The West River and Deerfield basins are graded to the level of this higher and older erosion surface, but farther north a chain of peaks including Stratton and Okemo swing eastward towards Ascutney. They appear to have formed a divide on this ancient land, as they do today; and beyond their crests rivers have run to the Saint Lawrence and Hudson basins from a time which antedated any of the familiar features of the New England landscape.

Although this flat upland surface is more complex than it appears to the eye, it dominates all of southern New England, and ramifying arms of it penetrate northward into the White Mountains of New Hampshire and Maine. Another great arm passes west of Mount Greylock and spreads out between the Catskill Mountains and the Adirondacks. During the long period of erosion when it was formed, New England was reduced nearer to the grade of the main rivers than at any other time either before or since, and only rocks which have effectively resisted all later assaults by the geologic processes of destruction surmount the surface. To the eye, the region appears so nearly planed that it has been called the New England peneplane.

The upland continues southward through the Berkshire and Litchfield Hills, descending in a series of almost imperceptible steps towards Long Island Sound and the Atlantic. A few miles south of Litchfield, Connecticut, its low angle of declivity increases abruptly, and the more steeply inclined surface passes beneath the waters of Long Island Sound. The sudden change in dip suggests that two erosional planes are present and that each was formed under somewhat different circumstances and in different periods of geologic time. The soundness of this surmise can be demonstrated in Long Island where sediments laid in a Cretaceous sea rest upon the older and more sharply inclined erosional plane and rise approximately to the level of the New England upland. The deposits form a wedge between the two planes, and their Cretaceous age supplies a series of dates that would otherwise be difficult to establish in New England's geological history. Erosion fashioned the New England upland in the early and middle epochs of the Tertiary period, immediately following the deposition of sediments in the Cretaceous sea. And the southward sloping plane upon which those sediments rest records an even earlier episode of denudation--an episode lost in the shuffle of later events in Massachusetts but preserved in fragmentary form in Connecticut, thanks to the protection afforded by the sedimentary cover.

Had we lived in central New England when erosion of the upland and of the younger straths was in progress, we would have noted that the valley forms were well defined in the headwaters and lower reaches of the streams, which made their way through a country of light-colored or gray clayey soil. In the middle reaches the valley boundaries were blurred and indistinct, and the country through which they flowed was surfaced by red and sandy soil. The middle region is now the lowland, but even then it formed a depression athwart the topographic and hydrographic features of the country; and its distinctive red soil resembled alluvial wash or fill in a long basin. Its low relief would have been as impressive in early Tertiary time as its higher relief is today, for then it had little topographic competition anywhere between the present sites of New Haven, Connecticut, and Northfield, Massachusetts.

The land had one dominant characteristic--a relatively flat or faintly terraced surface. But this surface concealed a mosaic made of an infinite variety of rocks, each responding to the attack of weather in its own particular way. Erosion has brought out the pattern of the mosaic, and we have retraced the steps in its development. Viewing the evolution of the countryside in retrospect, we see its features take form much as a worker on an inlaid bronze might watch the design come out when it is etched. The creation of the mosaic or inlay is another part of the history, and the relief of the land now permits closer scrutiny of the pattern than would have been possible in Cretaceous time.

The Mosaic of Central Massachusetts

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.

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.

Pl. 4. Rocks of the Triassic basin and their record.

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.

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.