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Bedrock in the upper portion of the Valley consists primarily of Precambrian igneous lava flows, shales, sandstones, and igneous and metamorphic crystalline formations. Bedrock of the lower portions of the Valley consists of marine sandstones, shales, and limestones that were deposited during the Cambrian and Ordovician periods. Outcroppings southward through the Valley indicate that bedrock formations are progressively younger.

Glacial deposits consist mainly of undifferentiated till that was deposited as the last glacier melted about 10,000 years ago. This glacial action was very important in forming the current features of the landscape. Especially important and characteristic among these glacial deposits are the extensive sandy soils associated with the bed of Glacial Lake Grantsburg in Burnett and Pine counties, and the numerous prairie pothole-type wetlands that occur in Polk, St. Croix, and Washington counties.

One important result of past glacial activity in the St. Croix River Valley is the highly productive soils that formed on the glacial outwash. Through the combined action of climate, vegetation, and proper acidity of these glacial deposits, soil developed that are very important for agricultural production. Productivity of all soils in the Valley is, of course, not equal. Soil conditions range from the sandy, acidic beach-derived soils associated with Glacial Lake Grantsburg, to deep, rich loams and silt loam prairie soils of the lower Valley. Productivity of these soils for wildlife appears to correlate nicely. For example, the acidic soils of Glacial Lake Grantsburg are ususally associated with relatively sterile Jack Pine Barren forest which supports limited wildlife populations. On the other extreme, the deep, rich prairie soils of the southern regions produce exceptional agricultural crops and also support large and highly diverse wildlife populations.