Final answer:
U-shaped mountain valleys in the Sierra Nevada are indicative of glaciation, where glaciers carved out broad valleys leaving behind a U-shaped cross-section. These valleys are distinct from those formed by rivers or from catastrophic flooding, showing the powerful role of glaciers in shaping landscapes.
Step-by-step explanation:
U-shaped mountain valleys like those found in the Sierra Nevada are indicative of glaciation. Glaciers are powerful agents of erosion that can carve out broad valleys with U-shaped cross-sections. Unlike valleys eroded by rivers which typically have V-shaped profiles, when a glacier moves through a mountain area, it tends to widen and deepen the valleys through physical erosion and abrasion, leaving behind the characteristic U-shape once the glacier retreats. Chemical weathering and catastrophic floods may also influence the landscape but are not primarily responsible for the creation of U-shaped valleys.Yosemite's Half Dome and other rugged terrains in the Sierra Nevada formed through a combination of geological processes including the subduction of tectonic plates, volcanic activity, and erosion. The region's topography that we observe today, characterized by U-shaped valleys and striking landforms, is a more recent development within the past 5 million years. Plus, these landscapes have been sculpted and altered significantly by glacial movements during the last ice ages.In conclusion, the distinct U-shaped valleys in the Sierra Nevada are a result of glacial activity that has shaped the landscape over millions of years. While volcanic activity and tectonic forces laid down the initial groundwork for the mountains, it was the relentless force of glacial ice that ultimately carved out the valleys as we see them today.