Here's why:
* Participial phrases are groups of words that act like adjectives, modifying nouns or pronouns. They contain a participle (a verb ending in -ing or -ed) and any related modifiers or objects.
* "Riding a horse" modifies the noun "sheriff," telling us how the sheriff was positioned.
* The participle is "riding," and it's followed by its object "a horse."
Let me know if you'd like to practice with other sentences!