London Bridge was built of stone for the first time. It was supported by a series of stone arches standing on small man-made islands. It had such a width that a row of wood houses and a chapel was built on top of it. In the spring it was impassable by ships because the flow of water under it varied in height on either side of the bridge by several feet at half tide. The bridge had the effect of slowing down the flow upstream, which invited wherries and rowboats and stately barges of the nobility. In winters in which it froze over, there was ice skating, ice boating, and fishing through holes in the ice. Outside each city gate were clusters of ragged buildings, small monasteries and hostelries, groups of huntsmen's kennels, and fencing schools. Outside one of the gates, a horse market was held every week. Horses wore horseshoes made of iron or of a crude steel. From the southwest gate of the city along the north river bank toward Westminster, there was a gradually extending line of rich men's mansions and bishops' palaces. On the southern bank of the Thames River was growing the disorderly suburb of Southwark, with fishermen's and boatmens' hovels, and taverns and brothels that were frequented by drunkards, rakes, and whores. On the north side of the city was a great forest with fields and wells where students and other young men from the city took walks in the fresh evening air. In some fields, country folk sold pigs, cows, oxen and sheep. Mill wheels turned at various streams. Near London in the country was a glass factory. At sunset, the gates of London were closed for the night. All taverns had to be closed, all lights put out, and all fires banked or covered when the bell of the church of St. Martin le Grand rang at 9:00 p.m. Anyone found on the streets after this curfew could be arrested. Gangs of young nobles or gangs of thieves, cutpurses, and looters roamed the streets after dark and sometimes rioted. Offenders were often beheaded and their heads placed on spikes on London Bridge.