St. Ninian's Tombolo
St. Ninian's Isle in the Shetland Isles provides a classic example of a tombolo, or ayre—a short spit of sedimentary material that connects an island to a nearby land mass or mainland. A tombolo is formed by waves curving around the back of an island so that they deposit sediment on a neighboring land mass, at the point directly opposite the island. Over time, these sediments gradually build up into a tombolo, which typically projects at right angles to the coast and has a beach on each side. St. Ninian’s Tombolo has been in existence for at least 1,000 years, and its permanence may be due to a cobble base underlying the sand. This tombolo tends to become lower and narrower during storms as a result of destructive wave action, while during calmer weather the waves build it up again with sand carried from offshore or the nearshore. The sediment that forms a tombolo may come from the mainland, the island, the sea floor, or a combination. Scientists have deduced that a tombolo will usually form when the ratio between an island’s distance from shore and its length parallel to the shore is less than 1:5 (for St. Ninian's Isle this ratio is less than 1). If the ratio is greater than 1:5, a feature called a salient may form—a sand spit that reaches toward the island but does not quite reach it.