The Rainy Lobe came from the northeast, pushing completely across the county. Its source area was the greenstone and granite area of western Ontario. Rainy’s ground till, though mostly obscure in the county, reflects those rocks in its composition.

In time the ice started to melt and the Rainy Lobe receded back across the county. The discharge channel for the melt water was dammed by another ice lobe advancing from the east northeast.

The ice advancing from the east northeast was the Superior Lobe. It moved across the southern half of the county from its source area in eastern Ontario and northeastern Minnesota. Gabbros and basalts were incorporated as sediments into the ice mass. This lobe also began to melt and receded back to the east, leaving till and moraines. The Mille Lacs – Highland moraines are representative. Mille Lacs Lake formed as the moraine trapped the melt waters there.

The St. Louis Sub-lobe of the Des Moines Lobe advanced into the county from the north. Dolomitic limestones and shales from Manitoba mark this lobe’s till. The St. Louis Sub-lobe pushed southwards covering the northern two thirds of the county. It overrode Glacial Lake Aitkin I and added those lake deposits to its sediment load. As this sub-lobe stagnated, moraines formed to the ice margins and melting ice created Glacial Lake Aitkin II and Glacial Lake Upham II, once again covering a large section of the county, trending from the west central border to the northeast. The Culver and Sugar Hills moraines are representative. The Wisconsin Glaciation ended approximately 11,000 years ago.

Glacial Lakes Aitkin II and Upham II were connected during much of their history. Melt water discharge to the south was blocked by stagnate ice and moraines. Initially Aitkin II drained via Upham II into the ancestral St. Louis River. The first outlet to open in the Aitkin II basin was the Snake River/Rice River channel. Later the Mississippi River outlet opened. Lake levels dropped, Aitkin II and Upham II separated, and the Snake River outlet closed. The lake was eventually drained.

The modern Mississippi River now meanders down the old lake bed

Oxbow lakes, locally know as “logans”, abound on the flat plain. The county’s peat resources have developed here. A site in Ball Bluff Township features lake deposits with fossilized snails. These date to approximately 10,000 years ago.

Glacial deposits dominate the present landscape. Terminal, recessional, and ground moraines are very prominent. Eskers and outwash fans can be found. The plain from Glacial Lake Aitkin is an unmistakable feature. Most of the county lakes have a glacial origin. Lakes formed by melting ice blocks are common, as are other lakes formed by damming and filling the depressions in moraines. Even the varied soil types and resultant vegetation depend on the composition of the parent till.

Basalt: A dark-colored, fine-grained igneous rock commonly found in lava flows. Basalt contains calcic plagioclasse and pyroxene (commonly augite).

End Moraine: Large ridge formed when the leading edges of a glacier advance pushed up large amounts of gravel and debris.

Esker: Narrow snake-like ridges formed by fast moving streams flowing in tunnels beneath the glacial ice. As the streams slowed, sediment came to rest filling the tunnels.

Gabbro: A family of generally dark-colored, medium- to coarse-grained igneous rocks formed at depth and consisting mostly of pyroxene and calcic plagioclase.

Graywacke: A loosely defined term referring to a dark sandstone or grit with angular fragments of quartz, feldspar and dark-colored rock, and mineral grains in a more fine-grained “clay” matrix

Ground Moraine: Glacial deposits that cover an area formerly occupied by a glacier. They typically produce a landscape of low, gently rolling hills.

Ice lobe: A restricted flow of ice away from an ice sheet.

Iron-formation: A chemically-precipitated, sedimentary rock consisting of silica and more than 15 percent total iron.

Metamorphic rock: A rock formed by the actions of heat and pressure. The alteration of an earlier formed rock generally by the formation of new minerals, but without the occurrence of melting.

Outwash: Stratified sediment washed out from a glacier by meltwater streams and deposited in front of an end moraine.

Shale: A laminated, sedimentary rock consisting mostly of clay-size grains.

Unconformity: A discontinuity in the succession of rocks containing a gap in the geologic record. A surface of erosion or nondeposition.