The formation of the Notch Valley was brought about by one of the successive terminal moraines flowing from the glaciated slopes of the ice-mountains farther northwestward, in the Adirondack region; while later the glaciated shoulders of Greylock’s Brotherhood slowly melted, eroding the slopes with small ravines in which the numerous rivulets flow to-day. The continental ice rivers from the higher glaciers northward apparently culminated in tremendous and successive cascades above Notch Valley, eroding the deep-cut gorges between Greylock and Ragged Mountains. The general directions of these currents, below these waterfalls, were various, finally leading down to the ancient Hoosac Lake, and flowing with it through the natural dam, northwestwardly, to the Hudson Valley, and thence to the sea. According to Professor T. Nelson Dale, an ancient lake six hundred feet deep existed in the Hoosac Valley ten thousand years ago. Perhaps ten times ten thousand years ago, a greater glacial sea overflowed the Hoosac Tunnel Mountains, leaving the bald summit of Greylock alone towering above the waves. As the terminal moraines of the great ice-sheet slowly receded, the various cascades formed pot-hole erosions, in their descent on the Canaan Hills, above the Connecticut Valley. Deerfield Arch was similarly formed by the force and chemical action of the eroding ice rivers, which flowed from glaciers, and wore through the wall of rock spanning the Deerfield Valley. Hawthorne compared this arch to “the arched entrance of an ancient church, which it might be taken to be, though considerably dilapidated and weather-worn.... It was really like the archway of an enchanted palace, all of which has vanished except the entrance—now opens only into nothingness and empty space.... This curiosity occurs in a wild part of the river’s course, and in a solitude of mountains.”[55] Dr. Wolfe says: “The summit of the arch and the water-worn pillars upon either side display ‘pot-holes’ and other evidences of erosion, and in the bed of the current lie fragments of similar attrite rocks which seem to indicate that at some period a series of arches spanned the entire space from mountain to mountain.”[56] Other erosions known as the “Twin Cascades” are found on the eastern slopes of Hoosac Mountain, above the eastern portal of the Tunnel, formed ages before the Hoosac Lake rippled in the “hollow vale” at North Adams. The Natural Bridge of the Mayunsook Valley is one of the greatest natural formations in Berkshire Highlands, and was also caused by erosions of the ice-currents ages ago.