What defines an overturned fold?

An overturned fold is characterized by one limb being tilted so much that it is turned over and effectively flipped upside down. This results in one side of the fold being oriented in the opposite direction from the other. The defining feature of an overturned fold is this extreme tilt, which is greater than that typically seen in more standard types of folds, such as anticlines and synclines, where the limbs remain relatively horizontal or gently sloped.

In contrast, the other options describe different types of folds or characteristics that do not apply to overturned folds. For example, while folds can have steep angles, the defining attribute of an overturned fold is not just the steepness of both limbs, but rather the inversion of one limb. Horizontal appearances and gentle slopes pertain more to other structural formations, not to the unique structure of an overturned fold. Thus, the intense tilting of one limb enables the classification of the fold as overturned, making this the correct distinguishing feature.