Sand Bar State Park takes its name from a natural sandbar between South Hero Island in Grand Isle County and the town of Milton on the Vermont mainland. The park is on the mainland, or eastern end, of that sandbar. The sandbar itself is the result of the Lamoille River, over tens of thousands of years, washing sediment downstream from the present Lamoille Valley. The river borne material sank to the bottom as the river emptied into the lake, eventually filling the lake to create the marshland south and east of the park, and the sandbar to the west. Natural lake depths here, without the sandbar, would be over 150 feet. As it is, water depth now along the top of the sandbar to South Hero is only a couple of feet. Because of the shallow water, the route along the sandbar served as a ford from the mainland to the islands well before construction of the first bridge in 1850. Crossing that first toll bridge, built of rock, gravel, and logs laid corduroy-fashion through the marsh and along the bar, must have been an adventure. Narrow and much lower than the present causeway, it was often flooded and always needed major repairs after damage caused by shifting ice each spring. Even crossing today's wider, higher causeway, completed in 1959, can be an adventure when snow blowing across the frozen lake blocks visibility, or spray and water from crashing waves washes across the highway during storms when the lake is high.