The ice sheet simply modified the pre-glacial topography; it changed symmetrical hills to asymmetric _roches moutonnées_ and left boulder till spread over much of the bedrock floor. The greatest changes were effected in the White Mountains, where the steep-walled river valleys were changed to troughs with a U cross-section, as in the scenic notches; or with steep headwalls like that in Tuckerman Ravine, a typical alpine cirque. Within the lowlands boulder till was left as a blanket, concealing the irregularities which were made in the rock floor at an earlier geologic date. These irregularities may pass unnoticed unless some construction project happens to reveal them. Bedrock is rarely over seventy feet down at any point in the lowland, but work at the Sunderland Bridge and the Coolidge Memorial Bridge encountered masses of glacial debris in a deep fluvial channel more than three hundred feet below the river surface and at least two hundred feet below the present level of the sea. This deep trough is not over one hundred yards wide, and if it were fully exposed to view, it would look like a miniature Saguenay gorge. Similar trenches in every part of eastern North America, from Hudson Bay to Cape Hatteras, show that the land once stood higher than it does now, and that the main rivers flowed in deep, narrow canyons, although the upland surface between the rivers had its present characteristics. Thus, in Pliocene time, while primitive members of the human race were entering old England, New England rose high above sea level, and its lowlands were trenched by quickened streams. The narrow gorges are an eloquent, if mute, record of rivers suddenly rejuvenated, their current accelerated and the exuberant waters cutting into freshly elevated rock. Massachusetts and the neighboring states along the Atlantic seaboard formed a plateau-like upland, perhaps one thousand feet higher than today, and the coastline lay fifty to one hundred miles out under the present waters of the Atlantic. The Pliocene episode of stream incision was of short duration. The gorges are not wide, and only near the sea do they cut deep into the coherent crystalline rock which gives New England its solid foundation. Nowhere did the land remain elevated long enough to permit the rivers to widen their canyons through the plateau-like country and to modify the essential features of the landscape. The latter were acquired in an earlier geologic epoch called the Miocene, and the scenic pattern carved by running water in that relatively remote division of time still dominates the region’s topographic form. _Rivers Carried Off the Everlasting Hills_