Complex sentence: Until 1868, when Runcorn Railway Bridge was opened, the only means of crossing the Mersey at or near Runcorn Gap were by fording or by ferry, with the lowest crossing of the river being the road bridge at Warrington.
Split into simpler sentences: Until 1868, when Runcorn Railway Bridge was opened, the only means of crossing the Mersey at or near Runcorn Gap were by fording or by ferry. Before that the lowest crossing of the river had been the road bridge at Warrington.