Years ago, I used a Garmin hand held GPS for such a situation.  With that unit you could create Waypoints for various locations such as good fishing spots etc.  With that unit you just pushed one of the buttons twice to create another point. 

I just laid the Garmin unit along the window sill in the tractor and could reach for it without actually looking at it.  I did this while using a rotary hoe, so I didn't want to take my eyes off the row.  I could do the whole process by "feel".

I later came back with my ATV and small trailer to hand pick the rocks.  This worked reasonably well but I haven't done it for a while.  This Garmin had a decent way of deleting the points as you worked an area so that you didn't repeat a point.  I did find that when I was navigating back to a point that I needed to glance at the Garmin and then back to where I was driving.  If I looked at the Garmin all the time I tended to get dizzy.