Ball-and-pillow structures are soft-sediment deformation features that form when denser, water-saturated sediment layers sink into underlying less-dense layers, creating rounded “ball” shapes. These structures are important indicators of rapid sedimentation and seismic activity and are often found in deltaic and turbidite deposits.
Reference: Lowe, D. R. (1975). “Water Escape Structures in Coarse-Grained Sediments.” Sedimentology, 22(2), 157-204.