The theory found on this website is accomplished with the help of the following literature: 

Meer, D. V. (2017). Impact on Granular Beds. Annual Review of Fluid Mechanics,49(1), 463-484. doi:10.1146/annurev-fluid-010816-060213

Thoroddsen, S. T., & Shen, A. Q. (2001). Granular jets. Physics of Fluids,13(1), 4-6. doi:10.1063/1.1328359

Katsuragi, H., & Durian, D. J. (2007). Unified force law for granular impact cratering. Nature Physics,3(6), 420-423. doi:10.1038/nphys583