A main clause also known as independent clause contains a subject and a verb with a complete express of thought. For example in the given -paragraph "Mason-Dixon is the subject and "designated" is the verb so it qualifies as main clause without this sentence would be incomplete.
An adjective clause is a type of clause the works to describe noun in the sentence. In the example "which is perhaps the most famous border ever established" explains the subject "Mason-Dixon line" and hence its the adjective clause.
A participle is a word formed from a verb it could be either past or present in the given example "is perhaps" and "established by surveying methods" qualify as participles.
