Complex sentence: A small dyke of porphyritic microgranite which has "a dog's-leg shape" in plan was intruded into these rocks during the Devonian Period; this formed a plane of weakness which controlled the position of the fault.
Split into simpler sentences: A small dyke of porphyritic microgranite which has "a dog's-leg shape" in plan was intruded into these rocks during the Devonian Period. This dyke formed a plane of weakness which controlled the position of the fault.