The Ozark Highlands area, covering nearly 47,000 square miles (122,000 km), is by far the most extensive mountainous region between the Appalachians and the Rocky Mountains.
The Ouachita Mountains to the south rise a few hundred feet higher, but are not geographically associated with the Ozarks.The Boston Mountains portion of the Ozarks extends north of the Arkansas River Valley 20 to 35 miles (32 to 56 km) and is approximately 200 miles (320 km) and are bordered by the Springfield and Salem Plateau to the north of the White River.Reef complexes occur in the sedimentary layers surrounding this ancient island.These flanking reefs were points of concentration for later ore-bearing fluids which formed the rich lead-zinc ores that have been and continue to be mined in the area.Much smaller pieces of the Ozarks extend westward into northeastern Oklahoma and extreme southeastern Kansas.
The Shawnee Hills of southwest Illinois, which lie near the eastern edge of this region, are commonly called the "Illinois Ozarks," but are generally not considered part of the true Ozarks.
The large, top most arc or bend in this part of the Arkansas River was referred to as the aux arcs—the top or most northern arc in the whole of the lower Arkansas.
Travelers arriving by boat would disembark at this top bend of the river to explore the Ozarks; the town of Ozark, Arkansas is located on the north bank at this location.
Drainage is primarily to the White River, with the exception of the Illinois River, although there also is considerable drainage from the south slopes of the Boston Mountains to the Arkansas River.
Major streams of this type include Lee Creek, Frog Bayou, Mulberry River, Spadra Creek, Big Piney Creek, Little Piney Creek, Illinois Bayou, Point Remove Creek, and Cadron Creek.
Many Ozark waterways have their headwaters in the uplands of the Boston formation, including the Buffalo, King’s, Mulberry, Little Red and White rivers.