Geologically and physically, the Berkshires are the southern continuation of the Green Mountains of Vermont, distinct from them only by their lower average elevation and by virtue of what side of the border they fall on. In physical geography, the Berkshires extend from the Housatonic River and Hoosic River valleys in western Massachusetts, to the Connecticut River valley in north-central Massachusetts, and to the foot of the lower Westfield River valley in south-central Massachusetts. In Connecticut, where they are referred to as the Litchfield Hills, they extend east from the upper Housatonic River valley in the northwest part of the state.