The Lake Superior region is a part of the southern margin of the great pre-Cambrian shield of northern North America. It is bordered and overlapped on the south by Paleozoic rocks of the Mississippi Valley and on the southwest by Cretaceous deposits. The pre-Cambrian rocks of the area, which may be divided into a considerable number of lithologic and time units, contain the great iron and copper deposits by which the region is most widely known. The great development of the mineral industry in this region has afforded the geologist unusual opportunity for study, as it has not only made the region more accessible but has justified larger expenditures for geologic study than would otherwise have been made. This fortunate combination of a field containing an exceptionally full record of a little-known part of the geologic column with the means of studying it has warranted the study of the pre-Cambrian with a degree of detail that has been practicable in but few other significant pre-Cambrian regions.
