Based on geomorphology, ages, and stratigraphy, the Marianna gap through Crowley’s ridge, a 13 km breach through a large‐scale unconsolidated horst in the Mississippi Alluvial Valley, as well as the transverse drainage that flows through the gap, began forming about 17 ka and were completely formed by 7–10 ka. Formation was driven by tectonic subsidence along part of the eastern margin of the Reelfoot rift, an aulacogen that has impacted development of the Mississippi Alluvial Valley region since the Precambrian. The breach occurred gradually, by subsidence‐driven fluvial erosion, and/or suddenly, by faulting. Our findings build on previous studies that suggest the eastern Reelfoot rift margin, as well as other nearby faults, have been recently active, evidenced by large‐scale sand blows emplaced in the Marianna gap area in the past 5–12 ky. We propose a mode of transverse drainage development that differs from previous modes in that it is driven by subsidence within a sedimentary basin, rather than uplift in an orogenic mountain belt. Our findings have implications for understanding both transverse drainage development in a sedimentary basin, and seismic potential of the Mississippi Alluvial Valley and other aulacogens worldwide.