Pieces of pottery and other artifacts show that Native Americans have lived along the shores of the Turtle River and Three Island Lake for thousands of years. Because pottery decoration and manufacture changed over time, archaeologists can date sites by the types of pottery, or wares, they find. Excavations in the park have uncovered three types: Brainerd, Blackduck, and Sandy Lake. Sandy Lake ware, dating from AD 1100 to 1750, was made with crushed mussel shell added to the clay; it is generally plain, though rims can be saw-toothed or wavy, and some vessels are decorated only on the lip with impressions. It is found from north-central Minnesota north to southern Canada and east to northwestern Wisconsin. Blackduck ware, dating from AD 600 to 1100, often has intricate bands of diagonal, linear, and circular impressions on the upper portions of vessels, many made with sticks and other objects wrapped with cord, and it is found from north-central Minnesota north to southern Canada and east across northern Wisconsin to Michigan's Upper Peninsula. Brainerd ware, dating from 1000 BC to AD 400, was given a fabric-like appearance by dragging a net or cloth across wet clay, or by leaving detailed net or cord impressions in the surface; it is found from north-central Minnesota west to the plains of Montana and north to Lake Winnipeg. Early Brainerd vessels often had pointed bottoms, which were likely propped in warm coals or embedded in sand or soft ground near the hearth, and over time rounded bases became more common.