Every stream has its load of sediment, as the silt- and sand-filled reservoirs along the edges of the valley so effectively testify. Each sandy river bed is an aggregate of rolling grains, moving with the current, slow where it is slow and faster where the current is accelerated, but travelling always towards the sea. Every grain is a piece of the countryside lost to the land and soon to become a part of the ocean floor. Very little of this sand comes from the lowland itself, for the Connecticut may cut the bank below Hadley, but it leaves almost as much sand as it acquires on the opposite shore. The river’s burden is brought to it by swift tributaries—the brook at West Pelham and hundreds more like it. Their sides are cut-banks, but no extensive sand bars are built to balance their erosive work; what they pick up they carry to the lowland, and what they bring to the lowland is soon transported to the sea.
The contribution which the tributaries make to the lowland rivers was demonstrated only too conspicuously by the great fans of coarse debris spread across the valley of the Deerfield River and the West River during the floods that accompanied the torrential rains of the hurricane. Parts of the village of Townshend, Vermont, nestling in the flat floor of the West River valley, were buried in gravel wash, and the hillside roads above were gullied ten feet deep. One harassed traveler aptly remarked that the original road level could be recognized from the few concordant remnants of pavement beside the trout brook.
The hill slopes at Townshend rise and end near Jamaica, about one thousand feet higher in elevation. Here the roads are in good condition. There are no signs of erosion, and the rolling uplands extend for miles with no signs of gullying or wash by the heavy rains.
The debris handled by the West River now and for ages past has come from the steep hill slopes along the main valley. Each load of sand has cut these slopes back from the main stream and has widened the lowland floor. So, for millions of years, the tributaries of the Connecticut have pushed the valley walls farther from the main river, and their tributaries in turn have pushed their hill slopes back, while the valley floors have steadily widened. The Connecticut Lowland was broadened in this way, and the tributary Deerfield has developed its valley in similar fashion but to a lesser degree. Today streams near the headwaters acquire sediment, not from the upland across which they flow to reach the deeply entrenched valleys, but from the steep slopes in the most remote recesses of the upland on which they rise.
Flat valley floors are broadened in coherent rocks as well as in unconsolidated sand—less rapidly, indeed, but just as surely; and every region is worn down to the grade of the streams which drain it, except for those rare masses of resistant rock which defy decay and yield reluctantly to their inevitable fate. The rocks of the Mount Holyoke and Mount Tom ranges, Mount Warner, the Pocumtuck Hills and the highlands on both sides of the Connecticut Valley are made of tougher ingredients than the lowland, and even millions of years of incessant onslaught by running water did not suffice to level them by Miocene time, when the lowland was excavated.
Pl. 3. Erosion remnants or monadnocks surmounting base levelled surfaces.
a. Mt. Sugarloaf, a remnant of Triassic rocks disappearing grain by grain down the Connecticut River.
b. Mt. Monadnock, a hill surmounting the New England peneplain, seen from Mt. Lincoln.