American Cliff Swallows build their gourd-shaped nests from mud, in large, overlapping groups that can reach to over 2,000 nests in a single location, often under bridges, or, as here, on cliff faces.

Modoc Rock Shelter is a rock overhang under the sandstone bluffs forming the eastern edge of the Mississippi River floodplain. It is significant for its archaeological evidence of thousands of years of human habitation.