The Precambrian igneous and metamorphic rocks exposed in the Minnesota River Valley between New Ulm and Ortonville are divided into three groups. The oldest is a basic complex of gabbro and diorite gneiss, quartz diorite gneiss, and garnetiferous quartz diorite gneiss. A few small isolated masses of serpentinized peridotite are included in the basic complex. The second group comprises varieties of granite and granite gneiss that are closely related and are assigned to the Minnesota Valley granite series. Five types are distinguished on the basis of textural and structural differences. The third and youngest group, referred to as postgranite intrusives, consists of several small and somewhat circular intrusives, a large number of basic dikes, and two small masses of late granite. Modes of the principal rock types are given.
The relative ages of the three major groups were determined by intrusive relationships, but close dating of the various rocks assigned to each group is not possible. On the basis of zircon types the Minnesota Valley granite series is assigned tentatively to late pre-Huronian or Huronian, and the postgranite intrusives are assigned tentatively to Keewenawan.