A product of ice and wind
Mount Sunday is a roche moutonnée, a specific type of hill sculpted entirely by glacial ice. During the Pleistocene epoch, the massive Rangitata Glacier flowed down this valley. The last major advance of this glacier reached its peak around 28,000 years ago during a period known as the Otira Glaciation. As the ice moved, it abraded the upstream side of the hill and plucked rocks from the downstream side, creating its distinct asymmetrical shape with a steeper, craggier southeastern face. The summit, which rises to an elevation of 611 meters, still shows the gouges and striations carved by the glacier. The hill itself is composed primarily of greywacke bedrock from the Mesozoic era.
The area is dominated by a powerful Föhn wind called the Nor'wester. This wind forms when moist air from the Tasman Sea is forced up and over the Southern Alps. The air cools as it rises, releasing its moisture as rain on the western side of the mountains. As the now-dry air descends on the eastern side, it undergoes compressional heating, becoming hot and dry. These winds can reach speeds up to 160 kilometers per hour, shaping the vegetation and contributing to the arid conditions of the Canterbury high country. This meteorological phenomenon is often signaled by a distinctive lenticular cloud formation known as the Nor'west arch.