Found this on another site...taken from 1959 magazine on pheasant populations: “Scientists, for the past 20-30 years, have endeavored to determine the capacity for pheasant production in this unusual South Dakota range. They have discovered that pheasants reach their greatest density upon areas most recently visited by continental icecaps or glaciers. Further investigation has shown that the most recent glaciated soils in Minnesota, Michigan, Illinois, Iowa, North Dakota and Wisconsin form the islands of greatest pheasant density within those states. Pheasant production is now associated with nutritional values of the most recently glaciated soils. And glaciers, according to science, deposited a greater variety of geology within South Dakota than any state east of the Rocky Mountains. It is an accepted fact that the Mankato and Cary glaciers, which ground away in abrasive action upon the bedrock limestone, transported into South Dakota huge deposits of pulverized lime. As the icecaps melted, the deposits settled to form the fertile soils of today’s grain factories. This glacier action formed the Limeline in South Dakota. The magic effects of this accessible lime to wild pheasants are as apparent as feeding lime products (seashells) to domestic chicken flocks in order to increase egg production. South of the Limeline, pheasants fail to reproduce naturally in such vast numbers. Egg infertility is given as the cause. North of the Limeline, winter storms destroy great numbers of birds required for the breeding season."