 Population pressure in the 12th and 13th centuries initiated the gradual clearing of the Weald, the forest spanning the borders of Surrey, Sussex and Kent, which had hitherto been left undeveloped due to the difficulty of farming on its heavy clay soil.Surrey's most significant source of prosperity in the later Middle Ages was the production of woollen cloth, which emerged during that period as England's main export industry.The county was an early centre of English textile manufacturing, benefiting from the presence of deposits of fuller's earth, the rare mineral composite important in the process of finishing cloth, around Reigate and Nutfield.