A rock, a big chuck of limestone, three miles long and a half-mile wide in Davenport, Ia., became an island about 20,000 years ago when the last glacier of the last ice age changed the course of the Mississippi River.
"Previously, the Mississippi turned to the southeast at the upper end of Rock Island County, perhaps 30 miles north of Rock Island and flowed southeast to what is now the Illinois River," according to Richard Anderson, a retired geologist who lives in Rock Island, Ill. "That course was blocked by glacial ice that came out of Lake Michigan Basin which is now Lake Michigan. That ice kind of deployed across the northeastern part of the state and blocked the old course of the Mississippi and diverted it past what is now Rock Island, the Quad City area."
The new course of the river cut a gorge through the limestone bedrock of the Quad City area. Within the gorge, the river split into two channels around what is now Rock Island. Anderson says among the hundreds of islands in the Mississippi, it is the only true rock island.
"Unlike most islands in the Mississippi which are louvial materials -- that is, deposited by the river itself as sandbars and silt and so forth -- these are constantly shifting, but Rock Island is a rock island and it stays where it is," Anderson says.
The Mississippi may have been home to many other rock islands in the past, but over the millions of years that it has flowed, the river has gradually washed them away. But Anderson says Rock Island is still around because it is in a relatively young part of the river.
"It's here because this segment of the river that flows past Rock Island, is a new section of the river. The river has flowed through this segment only for about 20,000 years."
The Sauk Indians gave the island its name: Assinnameeness, their word for rocky or rock island. The Sauk didn't live on the island, but they did use it as a setting for spiritual ceremonies.
When the United States Army took control of this area after the disputed Treaty of 1804, the island was recognized for its strategic value.
Gena Shantz, a local historian, says the Army decided the island was the perfect place to build a fort. "(There were) several reasons: one, the government was establishing a line of communication on the Mississippi River, which was essentially the western boundary of the United States at that time. Second of all, Rock Island was within three miles of the most notorious and hostile Indian tribe in the United States at the time and that were the Sauk."
Shantz is writing a biography of the first permanent resident of the island, George Davenport, who was brought in by the Army in 1816 to acquire supplies for the fort.
Davenport became a very successful entrepreneur and helped establish several towns including Davenport, Iowa in 1839. He was also involved in the effort to improve the country's transportation links. In the early part of the 19th century, most of those improvements involved making rivers easier to navigate or digging canals. But Davenport saw the potential of a transportation system that wasn't restricted to waterways.