Across the gorge of the Mississippi River lies the mouth of the Wisconsin River, where steeply notched valleys cut into sedimentary bedrock beneath level, plateau-like uplands and form abrupt bluffs along the valley margins. The layered rocks began as loose sediment on sea floors 450 to 550 million years ago and hardened into strata dominated by dolomite and sandstone, with fossil remains of marine life found throughout the area. These valleys began developing more than 1 million years ago, and melting ice sheets periodically released enormous volumes of water, sand, and gravel that scoured, filled, and shaped the valley profiles until about 9,500 years ago. Bridgeport and Prairie du Chein, Wisconsin, stand on terraces within the valleys that mark two different levels of sediment accumulation during these glacial meltwater floods. In 1673, a small band of French explorers led by Father Jacques Marquette and mapmaker Louis Jolliet traveled in two bark canoes down the Wisconsin River to this site and became the first Europeans to see Iowa. Lt. Zebulon Pike later evaluated the location for a fort in 1805, and the islands and pools in the Mississippi valley today result from ponding caused by the lock and dam system built in the 1930s. The valley now provides valuable habitat for many species of fish and wildlife and a dependable avenue of commerce for the region.