The underlying geology of Cambridge consists of gault clay and Chalk Marl, known locally as Cambridge Greensand, partly overlaid by terrace gravel. A layer of phosphatic nodules (coprolites) under the marl were mined in the 19th century for fertiliser. It became a major industry in the county, and its profits yielded buildings such as the Corn Exchange, Fulbourn Hospital and St. John's Chapel until the Quarries Act 1894 and competition from America ended production. The River Cam flows through the city from the village of Grantchester, to the southwest. It is bordered by water meadows within the city such as Sheep's Green as well as residential development. Like most cities, modern-day Cambridge has many suburbs and areas of high-density housing. The city centre of Cambridge is mostly commercial, historic buildings, and large green areas such as Jesus Green, Parker's Piece and Midsummer Common. Many of the roads in the centre are pedestrianised. Population growth has seen new housing developments in the 21st century, with estates such as the CB1 and Accordia schemes near the station, and developments such as Great Kneighton, formally known as Clay Farm, and Trumpington Meadows currently under construction in the south of the city. Other major developments currently being constructed in the city are Darwin Green (formerly NIAB), and University-led developments at West Cambridge and North West Cambridge, (Eddington). The entire city centre, as well as parts of Chesterton, Petersfield, West Cambridge, Newnham, and Abbey, are covered by an Air Quality Management Area, implemented to counter high levels of nitrogen dioxide in the atmosphere. The city has an oceanic climate. (Köppen: "Cfb"). Cambridge currently has two official weather observing stations, the National Institute of Agricultural Botany (NIAB), about north of the city boundary near Histon, and the Cambridge University Botanic Garden, about 1 mile south of the city centre. In addition, the Digital Technology Group of the University's Department of Computer Science and Technology maintains a weather station on the West Cambridge site, displaying current weather conditions online via web browsers or an app, and also an archive dating back to 1995. The city, like most of the UK, has a maritime climate highly influenced by the Gulf Stream. Located in the driest region of Britain, Cambridge's rainfall averages around per year, around half the national average, The driest recent year was in 2011 with  of rain at the Botanic Garden and at the NIAB site.