Aitkin County occupies over 1.2 million acres in east central Minnesota. Approximately nine percent of that surface is covered by water. There are over 200 lakes larger than 40 acres. The largest, Mille Lacs Lake, has over 60,000 acres within the county. The topographic relief is mostly gentle with glacial features dominating the landforms. The high point, Quadna Mountain near Hill City, is 1589 feet above mean sea level. The elevation of the unnamed low point in the southeastern corner of the county is 1083 feet. The Mississippi River meanders from the northern border towards the southwest and most of the county lies within its drainage basin.

Bedrock

Glacial deposits cover most of the county and rock outcrops are scarce. Depth to bedrock varies from over 300 feet in the northwest and west to less than 100 feet in the southeast and central areas. This underlying bedrock consists primarily of ancient formations from early and middle Precambrian time, and can be roughly divided into three major rock units.

The oldest is the McGrath Gneiss. It belongs to the Archean eon, early Precambrian, and dates from at least 2.7 billion years ago. It covers the subsurface in the southern quarter of the county. Outcrops can be found southeast of McGrath and west of Pliny. Outcrops are also reported south of Arthyde and along the Snake River near the southern county line.

Metamorphosed sedimentary rocks, iron formations, and minor volcanic rocks of the Mille Lacs and North Range Groups are next in age. They belong to the Early Proterozoic eon, middle Precambrian, forming approximately 1.8 - 2.0 billion years ago. They occur in the south central part of the county. Included are continuations of the iron formations of the Cuyuna Range which trend into the county from the west. Interesting occurrences include an iron sulfide deposit delineated in Glen Township, and a diabase dike that crops out near the southeast end of Long Lake, also in Glen Township.

The Animikie Group underlies the northern half of the county. It is younger than the previous groups, but still a member of the Early Proterozoic eon. This group is comprised primarily of graywacke, siltstone, and shale. A Cretaceous deposit of poorly lithified siltstone sits unconformably on this group in the northwest.

Glacial History

The Pleistocene Epoch – the Ice Age – began about 1.8 million years ago. Aitkin County experienced four major episodes. But only the last one, beginning about 75,000 years ago, left its mark. The features of the earlier glaciations are mostly obscured, either eroded away or buried beneath later deposits.

This final episode is referred to as the Wisconsin Glaciation. The ice advanced and retreated many times. Distinct lobes formed along the ice front in response to pressures within the glacier. In Late Wisconsin time, three of these ice lobes formed