Cretaceous sedimentary rocks , the next oldest unit, consist primarily of poorly lithified shale, sandstone, and limestone. The Cretaceous rocks in central Minnesota occur mostly as scattered outliers or erosional remnants on top of older Precambrian rocks, but become substantially thicker to the south and west. Paleozoic sedimentary rocks , mainly restricted to southeastern Minnesota, consist of interlayered sequences of sandstone, shale, and limestone that decrease in thickness to the northwest of the Twin Cities. Paleozoic rocks are not present in most of central Minnesota, but are very important aquifers in the Twin Cities and southeastern Minnesota. The counties fringing the northwestern metropolitan area are sites of recharge for these aquifers of the Twin Cities.
Precambrian rocks are the first bedrock encountered in most of central Minnesota.
These rocks occur along what is known as the midcontinent rift, and in central Minnesota consist primarily of basalt flows overlain by clean sandstones, which are very similar to overlying Paleozoic sandstones. The next oldest subdivision of the Precambrian, the Early Proterozoic , consists of a wide variety of rocks ranging from slate, schist, and gneiss to granite and gabbro. The northern extent of these rocks is roughly outlined by the Mesabi and Cuyuna iron ranges to the north, and they continue south and southeast beneath Cretaceous, Paleozoic, and Keweenawan rocks. The oldest Precambrian rocks, Archean , are made up primarily of bands of various types of granites situated between linear greenstone belts, and in west-central and southern Minnesota, gneiss. As a result of this weathering the minerals in the original rock have broken down to produce a mixture of mainly green and white clays and angular residual crystals that consist mostly of quartz.