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._ [Illustration: a. _Mt. Sugarloaf, a remnant of Triassic rocks disappearing grain by grain down the Connecticut River._] [Illustration: b. _Mt. Monadnock, a hill surmounting the New England peneplain, seen from Mt. Lincoln._] [Illustration: Fig. 5. _Block diagram showing the main features of central Massachusetts during the excavation of the lowland._] [Illustration: Fig. 6. _Block diagram showing main features of central Massachusetts after the Triassic basins were filled._] The lowland extends beyond our immediate region. It continues southward with diminishing elevation to New Haven, where it joins another broad depression, now flooded by the waters of Long Island Sound. _Before the Rivers Cut the Valleys_ Those who would see the land as it was before the rivers carved the lowlands must put back every grain of sand the waters carried away; they must fill in these valleys to the level of the Jamaica upland. Then only will the country be as it was before the streams were rejuvenated and started to cut deep trenches and to widen them as the Deerfield has done at Charlemont. 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.