The term grotto was applied to a cave or cavern made by hollowing out the ground, as at the Woodlands, or digging into a bank or hillside, as at Lemon Hill and Monticello. While some grottos were formed out of a naturally occurring cavity or depression—as in the case of the grotto at Belfield—they could also be created artificially, complete with contiguous artificial rockwork [Fig. 1] or constructed simply as a stone summerhouse. They could also be made from a combination of natural and artificial elements. In 1771, for example, Thomas Jefferson described building up a natural cave with rock or clay, then covering it with moss or thatch.
A general characteristic of the grotto was its ornamentation or embellishment with shells, rocks, and bits of glass or, as Richard Stockton wrote in 1767, curious “antiquities.” His wife, Annis Boudinot Stockton, planned her shell grotto at Morven, near Princeton, as a museum for the display of such ancient relics. Not only did Stockton visit Alexander Pope’s garden in Twickenham, near London, which was well known for its extensive shell-covered interior, but he also collected several souvenirs, including a piece of Roman brick from Dover Castle, a piece of wood from the King’s Coronation chair, and other curiosities for this grotto.
In his dictionary, G. Gregory repeats Ephraim Chambers’s definition of the grotto as an ornamented cave, adding the recipe for cement that would secure the placement of shells, fossils, crystalline minerals, and curious stones. This recipe also would render the grotto waterproof. Water was a key element in the creation of the grotto, whether it ran through the grotto as a stream or was available from a nearby spring. Accounts document that the sound and dampness of water were essential to the experience of the grotto.
Grottos were sited at the termination of walks, as recommended by Bernard M’Mahon, or in secluded parts of the garden as was the grotto at Economy, Pa. [Fig. 2]. They were sometimes built under another structure such as a summerhouse or a glasshouse—as at Belfield [Fig. 3]—or as an enhancement to a spring or ice house [Fig. 4]. The reports of visitors echo the experience of an enchanted, unearthly ambience in the grotto, invoking its association with the underworld and fantasy.
The grotto’s “veiled entrance with tracery of creepers,” and its interior “cool with greenish-light,” suggested links to the mythological tradition of an underworld deriving from classical precedents or caves as sites for mysteries and transformations. 
- Karen Bescherer Metheny et al., “Method in Landscape Archaeology: Research Strategies in a Historic New Jersey Garden,” in Landscape Archaeology: Reading and Interpreting the American Historical Landscape, ed. Rebecca Yamin and Karen Bescherer Metheny (Knoxville, Tenn.: University of Tennessee Press, 1996), 8.
- Naomi Miller, Heavenly Caves: Reflections on the Garden Grotto (New York: G. Braziller, 1982), 11–12.