with gradients around 40 feet/mile (8 meters/km) flowing through steep-sided valleys, notably the Westfield and Deerfield rivers and their larger tributaries. Mills were built to exploit the kinetic energy of falling water, and mill towns grew up around them, or company towns integrating production, residential and commercial activities. The development of steam engines to free industrialization from reliance on water power brought about the so-called Second Industrial Revolution when railroads were built along the rivers to take advantage of relatively gentle grades over the Appalachians. And so as hilltop farming towns declined in importance, industrial towns in the river valleys rose to local prominence. The Quabbin and Quaboag regions In northern Massachusetts, the higher altitude area to the east of the Connecticut River Valley is known as the North Quabbin region. These northern municipalities include Warwick, Orange, Petersham, Phillipston, Wendell, New Salem, and Athol near the New Hampshire border. The South Quabbin region (formerly the Swift River Valley) includes the towns of Barre, Belchertown, Pelham, Ware, Hardwick, Leverett, and Shutesbury. This area once included the four "Lost Towns" of Enfield, Dana, Greenwich, and Prescott, which were destroyed to make way for the Quabbin Reservoir. Farther south, the area called the Quaboag Hills includes Hampden, Monson, Wales, Warren, Holland, and Wilbraham on the Connecticut border. Numerous other towns stretching east towards Worcester are sometimes included in the Quaboag Valley region. Geology is similar to the Hilltown-Berkshire uplands with resistant metamorphic rocks overlain by thin and rocky soil. With less relief, the river valleys are less pronounced, but still moderately high gradient. The Quaboag Hills and Valley, the Quabbin Regions, and populated places stretching east towards Worcester are all locally known as "Hill Towns"; a term interchangeable with the Hill Towns west of the Pioneer Valley. Geology The mountain range in Berkshire County at the western end of Massachusetts is conventionally known as the "Berkshires". Geologically, however, the Berkshires are a westward continuation of uplands west of the Connecticut River and a southern extension of Vermont's Green Mountains. The Hilltown-Berkshire upland ends at the valley of the Housatonic River which flows south to Long Island Sound, and in the extreme north west of Massachusetts at the Hoosic River, a tributary of the Hudson. From these valleys, uplands to the east appear as a rounded mountain range, rising some although they are actually a plateau. West of the Housatonic-Hoosic valley system rises the