Complex sentence: On 6 August 1911 "Cornwall" ran aground on Pinnacle Rock, off Cape Sable Island in Nova Scotia while assisting the protected cruiser, which had also run aground.
Split into simpler sentences: On 6 August 1911 "Cornwall" ran aground on Pinnacle Rock, off Cape Sable Island in Nova Scotia. At the time, she was assisting the cruiser, which had also run aground.