Final answer:
Hanging valleys in Yosemite National Park were formed by the erosional forces of glaciers, which carved the main valley deeper than the tributary ones, causing streams to drop precipitously from the tributary valleys to create waterfalls.
Step-by-step explanation:
Hanging valleys such as those where the highest waterfalls in Yosemite National Park occur are a product of glacial activity. During the Last Glacial Period, glaciers carved out the main Yosemite Valley, which was deeper than the smaller tributary valleys.
As the ice retreated, these tributary valleys were left hanging above the main valley, leading to the creation of waterfalls where streams flow over the edge of the hanging valley into the main valley below.
The geological processes that formed these features include glaciation, erosion, and differential ice cutting. The main valleys were cut deeper by larger glaciers, while smaller glaciers in tributary valleys did not erode as much rock.
When the glaciers melted, the tributary valleys, now hanging above the main valley, resulted in steep drops for the streams and rivers flowing from them.