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.