The builders brought Quartz in to incorporate into the walls.

Not a typical farmer's field-clearing technique ;-)

Good enough evidence for me to think it could well be ancient.

http://www.neara.org/Muller/stonerows.htm

" Then, in October 1998, I guided Bill Sevon, a geologist with the Pennsylvania Geological Survey, through the site to get his perspective on how some of the large boulders were formed. When he saw the large quartz rocks incorporated in some of the stone rows and other features, he remarked that they could not have come from the ridge site itself, which consisted wholly of granitic gneiss, but must have been gathered somewhere in the Hardyston Formation in the valley below, a mile or more away (Buckwalter 1957).

The quartz pieces that have been found in many of the features represented in Fig. 1 are very much alike, in that all seem to have two flat, parallel faces, and vary from 3-6" thick (Fig. 2). One slab on top of the North Row measured 14" x 10" x 4"! And another in a short row between the South Row and the Terrace was 18" across; most, however, were 6-8" in diameter. Given the fact that all share the same general physical characteristics, it would appear that they all came from the same location, which was perhaps a large exposed seam of quartz where pieces could be easily pried out. The early settlers hardly would have bothered to gather quartz from a distant location to incorporate it in a wall, when their purpose for building walls was to rid their fields of stone. Quartz, however, had a symbolic and religious importance to Native Americans, not only because of its light, translucent color, which could have represented the brightness of the sun and moon, but also for its piezoelectric properties. With the discovery that the quartz came from the valley, the colonial hypothesis became a dead issue."