Geology
The bedrock of the park is basalt formed by volcanic activity 1.1 billion years ago. This is overlain by a thick layer of glacial debris. During the Wisconsin glaciation 16,000 years ago a small glacial lobe branched northeast off the Des Moines Lobe, blocking drainage from farther north. Water backed up into Glacial Lake Grantburg. The soil in the park is quite sandy from the sediments that accumulated in this now vanished lake. At the end of this ice age 10,000 years ago, meltwater flowing out of Glacial Lake Duluth carved the St. Croix River Valley. Today the river is one hundred times smaller than its glacial maximum. The ancient bank of the river is a bluff running through the park, well back from the current riverbed.
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