Mary's rock precariously balanced in a crevice between two of the standing stones at Pentre Ifan cromlech, near Newport, in West Wales. (It fell out right after I took this shot.) It's backlit by late afternoon sun; 28 September, 2011.

Mary's fast-eroding petrograph pressed into a knot of ivy clinging to an oak tree on the grounds of Ty Newydd, the National Writers' Centre for Wales, where I taught in late August / early September, 2011.

One of Mary's petrographs in sea grass on the coast of Harrington, Maine. July, 2011.

Mary's petrograph at Conques Abbey in Southwest France. Pictured with an 11th century column capital of Conques' original masons. May, 2011.

Another image from Conques Abbey, May, 2011.

Mary's rock on the cobbles in the place in front of the Abbey, shaded by my shadow. May, 2011.

The petrograph held in my hand, submerged in a travertine fountain in Conques' former cloister, with sunlit coins in the background. May, 2011.

The same Mary petrograph in a water-and-pebble-filled canning jar on the balcony ledge of Verkamps Store; twilight on the South Rim of the Grand Canyon, January, 2011.

Mary's preserved rock on the South Rim of the Grand Canyon, January, 2011.

A very similar image.

The jarred petrograph held in my hand on Verkamps' balcony at sunset, Grand Canyon. January, 2011.

Another shot from Verkamps balcony at twilight, South Rim of the Grand Canyon. January, 2011.

A glimpse of the rock in a rind of setting sun, Verkamps porch, South Rim of the Grand Canyon. January, 2011.

An earlier attempt at photographing the petrograph in a glass on the edge of the South Rim, with the Canyon beyond. January, 2011.

A similar image, taken on Verkamps porch. January, 2011.

A necklace of petrographs printed with Mary's image--most of them taken when we were in college, c. 1982--set into a stream in Dorloo, New York, on a farm in the Leatherstocking District. May, 2010.

One of the petrographs in the Mill River, Northampton, Massachusetts. Summer, 2009.

One of the Mary petrographs in the Mill River farther upstream in Haydenville, Mass. Summer, 2009.

Another version of the "flames" image from the Haydenville stretch of the Mill River, Summer, 2009.

A string of Mary petrographs set in a river in Chester, Vermont, August, 2009.

Another Chester, Vermont image of petrographs in dry and wet river sand. I find this one to have a curiously formal aspect, like a series of 19th century miniatures. August, 2009.

One of the first Mary images I made; Mill River, Northampton, Massachusetts. Summer 2009.

Another Mary image from the Mill River in Northampton, Summer, 2009.

Also from the Mill River in Northampton, Summer, 2009.

A line of Marys: Mill River, Northampton, Massachusetts. Summer, 2009.

This one was from the Mill River in Haydenville, Mass., Summer, 2009.

This one was also taken in Haydenville, Summer, 2009.