At first glance, it looks like an elaborate sand fort for an imaginary pirate. But instead of a Jolly Roger’s skull and crossed bones, the banner rippling from its driftwood staff is a small American flag. A closer look at the structure, which appeared at the end of Fuller street beach in Edgartown last week, reveals a low, horseshoe-shaped enclosure made of sand, stones and beach peat.

Courage, Salt Water Cures It All and Be the Good are among dozens of encouraging words and phrases that have been carefully inscribed on beach stones and placed inside the enclosure, which opens to the water side. Holstered in the sand, permanent-ink markers invite passersby to add their own rock-thoughts to the colorful heap, or to inscribe a stone and take it along.

A passerby adds to the collection.

Louisa Hufstader

A passerby adds to the collection.

Louisa Hufstader

The Kindness Rocks Project was started by Megan Murphy of Barnstable (thekindnessrocksproject.com) who writes on her site: “During difficult or stressful moments in my life, I found myself looking for signs on my morning walk, such as a heart shaped rock or a piece of sea glass. I perceived this sign as a divine message or the random inspiration I needed to signify that things would be o.k.”

Ms. Murphy began painting rocks with inspirational phrases and leaving them in public places. Strangers responded warmly to the random acts of encouragement, and she created the first public project on a Barnstable beach in August 2015. The idea caught on quickly.There are now scores of kindness rock gardens across the continent, in the Caribbean and overseas.

According to a Nov. 1 post with photos on the Facebook page, the Edgartown installation was created by a couple named Michelle and James.

Edgartown’s rock garden does not appear on the interactive map of sites, perhaps because it’s likely to begin washing away soon as the increasing moon draws higher tides ashore. Until then, anyone may pocket a stone’s worth of kindness by walking a few steps north on Fuller street beach.