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For anÌýevolutionary biologist, a trip to the Labyrinth Canyon Wilderness brought an added discovery

My goal on this hike was to photograph a series of inaccessible alcoves high on a wall in the Labyrinth Canyon Wilderness on the Colorado Plateau.

I returned with a photo, but the most vivid memory was the joy of finding a floral display in a hanging garden in an easily accessible alcove.Ìý

Alcoves are prominent features of the Colorado Plateau, a high plateau centered on the Four Corners area and drained primarily by the Colorado River and its major tributaries, the Green, San Juan and Little Colorado Rivers.

Alcoves come in all shapes and sizes, and their appeal is not only in their forms but also in the possibility that they served as shelters or homes of Native Americans for thousands of years. Petroglyphs, pictographs and pigmented imprints of hands are frequently found where Native Americans camped or lived. Perhaps the most famous alcove in Colorado provides protection for Cliff Palace in Mesa Verde National Park.

Alcoves are formed by water moving through stone. Sandstone is porous, so water can seep directly through it, following the path of least resistance, dropping vertically or moving horizontally. When water seeping through sandstone encounters a less porous layer, or when it encounters an impermeable layer of shale, it is forced horizontally until it meets a steep or vertical surface.

At that point, sandstone may be shattered in slender layers when water freezes—a volume of water turns into a greater volume of ice (ice floats, right?) applying pressure sufficient to crack stone. In areas where the stone is constantly wet, water dissolves the calcium carbonate cementing sand grains together—slowly, the stone washes away. As the alcove grows deeper, layers of stone on the ceiling may exfoliate, or break in thin layers, gradually making the alcove taller.

Water moving laterally over an impermeable surface accumulates on the floor of the alcove, creating a spring and spring pool in the shady alcove when it is hot and dry outside.Ìý

As I approached the cliff from which I would photograph the series of alcoves, I encountered a shallow alcove, about 8 feet deep, 26 feet tall and 30 feet wide. I entered it for a shady respite but quickly noticed a green plant with pink flowers growing from a crevice in a vertical wall, about 15 feet above the floor.

As my eyes adjusted to the lower light, I noticed two shallow shelves, 1 foot and 3 feet above the floor each lined with flowers. Each of the shelves was also packed with lichens and appeared to be formed by ephemerally seeping water. I had stumbled into a hanging garden.Ìý

My colleague, Stacey Smith, identified the flowering species as the Navajo evening primrose, Oenothera caespitosa subspecies navajoensis, which is essentially limited to the Colorado Plateau. This species is self-incompatible, so it relies on pollinators.

Four species of evening primroses are found in the Four Corners area, and all of them open large, white flowers at dusk that are pollinated by hawk moths such as the white lined sphinx or the Vashti sphinx, plus several species of large solitary bees that fly at dawn and dusk. Evening primrose flowers last but one night, and shrivel, turning deep pink, by the middle of the following day. Some evening primroses are annuals or can be either annual or perennial, while Navajo evening primroses are perennials.

I might have thought that the plants in hanging gardens would be unusual, with special adaptations for living on vertical surfaces. That is not the case for Navajo evening primrose, for I also saw it around my campsite and in sandy areas.Ìý

I left the hanging garden and within a few minutes I was on the ledge with a view of the alcoves that I had seen the previous day, from miles away. Alcoves form readily in the top layer of sandstone—5 alcoves are visible, some shallow, others deep. The feature that now haunts me is that line of greenery, perhaps a species of shrub, spilling from the nearest alcove. The inaccessibility of the alcove will keep the details of its garden secret.

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