The abbey's two western towers were built between 1722 and 1745 by Nicholas Hawksmoor, constructed from Portland stone to an early example of a Gothic Revival design. Purbeck marble was used for the walls and the floors of Westminster Abbey, even though the various tombstones are made of different types of marble. Further rebuilding and restoration occurred in the 19th century under Sir George Gilbert Scott.

Answer this question, if possible (if impossible, reply "unanswerable"): What were the two western towers built from?
Portland stone