Final answer:
A massive pile of highly irregular debris forming a ridge or cone on a valley bottom is caused by a landslide. This process results from factors such as erosion, saturation by precipitation, seismic activity, and excess weight. Erosion and landslides together shape mountainous landscapes over time.
Step-by-step explanation:
The topographic feature described as a massive pile of highly irregular debris in the form of a low-lying ridge or cone on a valley bottom is a result of a landslide. Landslides include a range of ground movements like rock falls, deep failures of slopes, and debris flows. They can be triggered by various factors such as over-steepening due to erosion by rivers, glaciers, or ocean waves, saturation of rock and soil slopes by rain or snowmelt, seismic activity from earthquakes, and excess weight from natural or anthropogenic sources.
Erosion plays a significant role in shaping the terrain after mountain-building events. Ice, for example, is a potent force that carves and sculpts rocks, leaving sharp peaks and serrated edges. Landslides themselves contribute to this process by uncovering bare rock and creating new, often times irregular, landscapes. It is the combination of uplift, erosion, and mass-wasting events like landslides that continue to shape the environments in mountainous areas.