If you live in Arkansas, chances are you’ve heard of the Ozark Mountains. Actually, the correct geologic term is Ozark Plateaus. Unlike typical mountains in which the bedrock has been squashed and folded, the Ozarks are one broad dome-like structure made up of flat-lying sedimentary bedrock. The hills and valleys of the Ozark topography are the result of rivers carving into this dome, rather than compression or deformation.
The picture above was taken overlooking the Buffalo River. The various hills, from the foreground to the distance, are roughly the same height. Of course they are! If not for this and other rivers, the landscape pictured here would be one solid flat surface, as tall as the highest peaks in the picture, stretching to the horizon.
Zinc ore collected in 1943 from the Rush Creek Mining District, Marion County, Arkansas. The brown mineral is sphalerite: an ore of zinc. The pink mineral is dolomite – it’s pretty, but not economically valuable. They were both deposited on the gray dolostone; you can just make it out on the right, in back.
Zinc deposits are found throughout northern Arkansas, commonly with the lead mineral, galena. They’re most abundant in Marion County, in a two mile stretch of rugged terrain, along Rush Creek, where 4 faults come together. That area was mined for lead and zinc in the late 1800s and early 1900s.
It’s typical to find rich ore deposits in rock that’s been fractured by faulting. The fractures facilitate migration of mineral-rich ground water which deposits the ore minerals in the fractures. It’s hard to see in the picture, but the fractured dolostone rock, in this specimen, is bound together by the sphalerite and dolomite minerals.