STREAM TERRACE DEPOSITS AND ALLUVIUM
Stream deposits (alluvium) are found in the valleys of the small streams around the Tower and, in particular, in the valley of the Belle Fourche River, that cuts across the southeast corner of the Monument. The deposits consist of unconsolidated gravel, sand, silt, and mud.
Along the Belle Fourche River, northwest of the river and between it and the main road, the river cut a terrace in the Spearfish formation. On the terrace were deposited gravel and sand.