## 3 Field Geological Investigations  
### Landslide Type  
The landslides in the study area can be classified into three main types according to the Varnes classification method, as updated by [PERSON] et al. [42]: flowslides, slides, and falls [7]. Flowslides typically occur in saturated soil layers with the slip surface located below the groundwater level. These landslides are often large-scale, characterized by long sliding distances and considerable fluidity, with the soil behaving almost like a fluid. Flowslides are particularly concerning due to their ability to transport large quantities of displaced materials over long distances. These materials, often including terrace deposits, contribute to significant damage to farmland, infrastructure, and property, and in some cases, result in casualties [43]. The vast scale of flowslides also makes them more dangerous, as they can trigger multiple failures across large areas, leading to the creation of new failure zones.  
Slides typically occur in homogeneous, dense clay layers and are generally smaller in scale compared to flowslides. Slides usually follow flowslides, particularly when the slope has already been destabilized by earlier flowslides. The slip surface of slides is shallow, typically above the groundwater level, with minimal internal deformation, typically involving single blocks of soil that move down the slope. Morphologically, slides are often characterized by a prominent main scarp, referred to as the \"head scarp\", and a back-tilted bench, which is the upper section of the slide. These features are typically stable for a certain period but can lead to further slope destabilization if subjected to additional forces, such as continued groundwater infiltration or external loads.  
Falls typically occur on steep cliffs formed by flowslides or slides. In the fall stage, loess blocks or soil masses separated from the main slope face descend rapidly, driven by gravity, and become highly mobile projectiles. The blocks involved in falls are usually large, solid, or fragmented materials that possess significant kinetic energy, especially when descending from steep, high altitudes. The speed and impact of these falling materials can further destabilize the slope and damage the surroundings, depending on the size and velocity of the falling mass. The sudden release of energy during a fall can also lead to secondary failures on the slope, further complicating the landslide process.  
These three types of landslides--flowslides, slides, and falls--often occur in succession at the same location, creating retrogressive landslide groups (Figure 2). This process typically begins with a flowslide, which initiates failure, followed by a series of secondary slides and falls. The progressive nature of these events leads to multiple stages of failure, and as each event unfolds, the failure zone expands, resulting in an increasingly unstable slope.  
Figure 1: Geological overview of the South Jingyang Platform: (**a**) study area; (**b**) Google Earth map and the stratigraphic structure map.  
This sequential occurrence of landslides is a key characteristic of retrogressive landslides, making the associated disasters more complex and severe.