Mile 170.4 - RBD Limestone Bluff Shelfs

John Ruskey

20 Apr 2026 � 1 min read

170.4 RBD Limestone Bluff Shelfs

This is the 2nd prominent outcropping of the Missouri Bluffs, the so-called St. Louis limestone, which is the predominant surface rock of the area. At low and medium water levels the river rubs against this unusual shelf of St. Louis limestone which can be approached from water and a landing made for a spectacular picnic spot with one of the last views upstream of the Arch. If your line of travel is along the main channel right bank descending stop and stretch your legs here for a spectacular last view of the Great Arch rising above the Anheuser Bush Brewery and industrial South St. Louis.

The floodplain along the Mississippi on the Illinois side is known as the American Bottoms, and extends from Alton down to the Kaskaskia River. Once the dominion of the river, now leveed and settled by generations of settlers, the landscape is still riddled by wetlands, swamps, and oxbow lakes.

Deforestation of the river banks in the 19th century to fuel steamboats had dramatic environmental effects in this region, leading to the Mississippi River between St. Louis and the confluence with the Ohio River becoming more wide and shallow, as unstable banks collapsed into the water. It resulted in more severe flooding and lateral changes of the major channel, causing the flooding and destruction of several French colonial towns, such Kaskaskia, Cahokia, and St. Philippe.

The southern portion of the American Bottom is primarily agricultural, planted chiefly in corn, wheat, and soybean. The American Bottom is part of the Mississippi Flyway used by migrating birds and has the greatest concentration of bird species in Illinois. The flood plain is bounded on the east by a nearly continuous, 200–300 foot high, 80-mile long bluff of limestone and dolomite, above which begins the great tallgrass prairie that covers most of the state. The Mississippi River bounds the Bottom on its west; the river abuts the bluffline on the Missouri side. Its maximum width is about 9 miles in the north, and it is about 2–3 miles in width throughout most of its southern extent. (Adopted from Wikipedia)

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