I've worked on documenting an old cemetery, so here's my two cents worth: first, do no harm, so - NO BLEACH! (these don't need it, but when else will I get the chance to say it). Jeff Gordon is spot on - if needed, use a soft brush that is softer than the stone - like a straw whisk broom - these are sandstone probably, so only very light cleaning if any. Bring a spritz bottle with plain water, and here is the winner: it may take multiple viewings at different times of the day to bring out the words - the angle and intensity of the light is a huge factor. Rubbings can degrade the stone, so I'd go with spritzing and taking pix. Sometimes you can see it on the picture better than with your eye. If you figure it out, let someone know so they are not lost to time. My husband's family has fieldstone headstones, but none are etched. So you have a cool find there.