Al, he finds burials, so marks the head to toe (or toe to head) of the grave. He senses the disturbance/shift in the earth, not necessarily the body or the stone. Many did not have stones, anyway.
He found 20 +/-3 burials. In the 1950s a group went around and wrote down all that was seen in each cemetery. This one had 10 stones. It is very common for a cemetery to have unmarked graves. Many times stones were unaffordable or even in this area, difficult to get. Portland didnt get their first stonecutter until 1800 (Bartlett Adams), so stones before that had to be sent away for and shipped. Most didn't have the money for that.