The morphology of the failure scar has been a long-debated issue concerning the Loess Plateau of China, and the lack of normative data has hampered vital research in this area. In this study, a series of laboratory experiments were conducted to observe failure geometries and volumes under rainfall simulations. The following six failure-scar types occurred: The sequence of the types described here may be same to that of the following text, namely Tf, Cd, Cu, Ia, Ps, and Co. The number of failure masses in the experiment for scar types Tf, Cd, Cu, Ia, Ps, and Co amounted to 45, 26, 23, 3, 2, and 1% of the total, and the corresponding volumes were 58, 16, 20, 2, 3, and 1% of the total, respectively. This implies that Tf, Cd, and Cu were the three major types of failure scars during the process of gravity erosion on a steep loess slope, and the scar Tf was the most significant. The sensitivity coefficient – the degree of variation in the target value caused by a change in a crucial factor when other conditions remain unchanged – of the slope gradient and rainfall intensity for the total volume of the Tf scar were 4.5 and 3.7, respectively. In other words, a relatively dangerous failure scar (Tf) might appear if the slope became steeper, or if the rainfall became more intensive. In addition, the sensitivity coefficients of the rainfall duration for the total volume of the Cu scar was significant, being 12.4. This shows that a long-duration storm could easily induce a large-volume failure with a Cu scar. The experimental results obtained here provide a morphogenic insight into the gravity-erosion control on a loess gully sidewall.