Walking along the Mill River conservation trail today there is little to suggest that this quiet valley — situated roughly between Pine Street and Pulpit Hill Road — has changed much in the three centuries since Hadley farmers first moved into the area. But a glance at some of the old maps tells a different story: “Factory Hollow” was once the scene of intensive use of the River to power early industry in Amherst, ranging from grist mills to substantial textile and paper mills. In fact, the River — along nearly its entire Amherst course from Leverett in the north …