Soft sediment deformation structures in lower Miocene alluvial sediments of the High Plains include three distinct types: long projections with nearly circular cross-sections, short projections with reniform cross-sections and massive volcanic ash structures. These structures project more than one meter downward into more dense host sand. The morphology of each type of structure is described and each is related to associated structures in the host sand. The origin of the long projections is probably related to activity of plants and/or animals. These projections probably formed by fining of burrows or root casts by fluid-saturated volcanic ash. The short projections intermediate axes are coincident with the axes of loaded ripples and indicate that these structures probably formed by thixotropic flow through ripple troughs. Volcanic masses within host sand are most difficult to explain. Laboratory experiments were conducted to duplicate these structures and suggest that they formed by movement of sand through a layer of volcanic ash. Collectively, these structures are unique and indicate probable formation in a braided stream environment with high fluctuation in discharge, biologic activity and rapid accumulation of sediment.