HARPSWELL — Whoever coined the famous phrase, “necessity is the motherhood of invention,” probably uttered it after learning how Maine highway engineers solved a nasty “bridge” problem on Route 24 between Orrs Island and Bailey Island.

Actually, there was no bridge problem until the state decided to extend the highway south through Harpswell in the late 1920s.

Route 24 actually linked the three islands (Sebascodegan, Orrs, and Bailey) that composed eastern Harpswell. For the engineers charged with getting Route 24 to Jaquish Gut at the southern end of Bailey Island, jumping the gap between Sebascodegan Island and Orrs Island appeared insignificant to spanning Will’s Gut between Orrs Island and Bailey Island.

Here, engineers faced strong currents and highly fluctuating tides that made a standard bridge unfeasible. From Orrs Island, the land plunged into the sea, but granite ledges extending north from Bailey Island might support a bridge.

After studying their options, engineers created the one-of-a-kind Cribstone Bridge by stacking granite slabs atop each other in a checkerboard pattern.

If the granite marked the white squares on the checkerboard, the openings left between the slabs marked the black squares, through which the sea could ebb and flow without exerting its dynamic pressure on the slabs.

Granite workers cut 12-foot slabs at a quarry in Pownal and loaded them aboard barges for the journey to Harpswell. The State Highway Commission (the forerunner to the Maine Department of Transportation) installed a railroad track for a large crane, which was used to lift the heavy granite slabs into position.

As modern motorists might realize, engineers designed the Cribstone Bridge to accommodate cars from the 1920s, not the 1990s. Narrow throughout its 1,200-foot length, the bridge makes a graceful curve toward Orrs Island. Two vehicles can pass in opposite lanes, but the distance appears tight.

Work crews never fastened the granite slabs together, but left them resting atop each other to shift minimally with the tide and the seasons. About the only steel in the bridge can be found in the 52-foot span that allows boats to pass beneath the structure’s midsection. The state attached a sidewalk to the bridge in the mid-1930s.

Although opened in 1928, the Cribstone Bridge will not be replaced in the near future. Its rare design (the German Luftwaffe took out a similar bridge in Scotland during World War II) has withstood increased tourist traffic, heavy trucks, and everything that modern society can throw at it.

The tide still surges through Will’s Gut, and the Cribstone Bridge still stands. For an approximate $120,000 investment in the late 1920s, Maine has received more than a fair return.

— By Brian Swartz