Physiognomy and Structure: This alliance is characterized by a mixed and variable physiognomy. Individual occurrences may be herb-dominated, shrub-dominated, or sparsely vegetated with vascular plants (though sometimes with dense patchy cover of mosses, hepatics, or lichens).

Floristics: This vegetation is characterized by Hydrangea arborescens with seepage indicators such as Impatiens pallida, as well as a wide diversity of vascular species. Overhanging tree species providing shade vary with location and rock chemistry. Associated shrubs may include Staphylea trifolia, Physocarpus opulifolius, Hypericum prolificum, and others. Herbaceous species are widely variable, depending on rock chemistry, and may include Aquilegia canadensis, Cystopteris protrusa, Deschampsia flexuosa, Dodecatheon meadia, Dryopteris marginalis, Heuchera americana var. hirsuticaulis, Parthenocissus quinquefolia, Pilea pumila, Polymnia canadensis, Toxicodendron radicans ssp. negundo, and many others. Sandstone substrates support non-calciphilic species such as Deschampsia flexuosa, Dennstaedtia punctilobula, Dryopteris marginalis, and Ribes cynosbati, while calcareous substrates support Adiantum pedatum, Aquilegia canadensis, Arabis laevigata var. laevigata, Aruncus dioicus var. pubescens, Asplenium platyneuron, Asplenium trichomanes, Cystopteris protrusa, and Pilea pumila. Seepage areas on calcareous rocks support Cardamine diphylla, Dodecatheon meadia, Heuchera villosa, Impatiens capensis, Impatiens pallida, Polymnia canadensis, Saxifraga virginiensis, and Thaspium pinnatifidum. Sheltered areas under rock overhangs or boulders may harbor thallose liverworts and various mosses, including Mnium spp.

Dynamics:  Near-constant seepage and highly irregular topography prevent the establishment of trees.