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. 
- Stockton, Richard, July 1767, in a letter to Annis Boudinot Stockton, describing England (quoted in Greiff 1989: 1:38)
- “England is not the place for curious shells, therefore you must not expect much by me in that way; but I shall bring you a piece of Roman brick, which I knocked off the top of Dover Castle, which is said to have been built before the death of Christ. I have also got for your collection a piece of wood, which I cut off the effigy of Archbishop Peckham, buried in the Cathedral of Canterbury more than five hundred years ago; likewise a piece from the king’s coronation chair, and several other things, which merely as antiquities, may deserve a place in your grotto.
- “I have dined with Mr. Neat several times. Miss Neat is exceedingly obliged for the shells you sent her. She has made some curious flowers out of them. She and her mamma are both engaged to find out for me the best cement for sticking shells in the large way, which I know will be needful for you when you begin your grotto. You see I do not omit attending to your commands. I told you in my last that I intended a visit to Twickenham, to see Mr. Pope's house, gardens, and grotto, for you direction. This I shall execute if it please God."
- Jefferson, Thomas, 1771, describing plans for Monticello, plantation of Thomas Jefferson, Charlottesville, Va. (1944: 26–27)
- “the ground above the spring being very steep, dig into the hill and form a cave or grotto. build up the sides and arch with stiff clay. cover this with moss. spangle it with translucent pebbles from Hanovertown, and beautiful shells from the shore at Burwell’s ferry. pave the floor with pebbles. let the spring enter at a corner of the grotto, pretty high up the side, and trickle down, or fall by a spout into a basin, from which it may pass off through the grotto. the figure will be better placed in this. form a couch of moss. the English inscription will then be proper.
- 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.