。
A rockfall blocked his path.
Literal
Rockfall [subject-が] he [possessive-の] going-way [object-を] obstructed.
行く手 ('the direction one is going / one's path ahead') is a set compound of 行く ('go') + 手 ('direction/way'). 手 in this sense is a metaphorical extension of 'hand' to 'direction / side.' さえぎる ('to block / intercept') is a transitive verb for obstructing motion or vision.