His plan was quickly and intelligently formed. Charleston lies upon a tongue of land between the Ashley and Cooper rivers which, uniting at the battery, form the bay exactly as the union of the North and East rivers does at New York. Indeed, a map of Charleston in outline differs from a map of New York only in one important particular, that the widening of the peninsula in its middle part occurs on one side in Charleston and on the opposite side in New York. South of the Ashley river the coast line sweeps in a semicircle trending northwards and bounded by James Island. That island is separated from the mainland by Wapoo Cut, which connects the mouth of the Ashley river with Stono Inlet. Stono Inlet separates James Island from John's Island and the mainland on the south, and its entrance is from the sea. On the eastern side of James Island there is an inlet known as Folly river which has many ramifications, and which in its general course cuts off from James Island a marsh known as Folly Island. This marsh lies along the ocean front very much as Sandy Hook does at the mouth of New York harbor. A narrow and shallow creek cuts Folly Island in two towards its northern end, and the space north of that creek is known as Morris Island. The northern end of Morris Island abuts upon the entrance to the harbor and commands it on the one side while Fort Moultrie on Sullivan's Island commands it on the other. Fort Sumter, built upon an artificial island in the middle of the harbor, forms the third point in the triangle in which the Federal fleet had encountered such difficulty, and suffered such defeat. General Gillmore's plan was to capture Morris Island, and gain possession of its northern extremity, known as Cummings's Point, where stood Battery Gregg, on guard against entrance to the harbor. By an error in judgment the Confederates gave him his opportunity. They withdrew from the marsh known as Folly Island, where they might have defended themselves almost to the end of time. Gillmore was quick to see their mistake, and to take advantage of it. He instantly seized upon the southern end of Folly Island, and set to work to convert his troops into amphibians. In boats, on rafts and by wading through indescribable mud he pushed them northward to Morris Island. In anticipation of this movement on his part, the Confederates had strengthened a battery on that island, and named it Fort Wagner. They had armed it with the best they had in the way of heavy guns, light guns and infantry forces of grandly desperate courage and determination. Here was to be fought out the question of the possession of Morris Island and the control of Charleston harbor.