Some quarrying of sand and gravel occurs in the St Albans area. In the past, clay has supplied local brick-making and still does in Bovingdon, just south-west of Hemel Hempstead. The chalk that is the bedrock of much of the county provides an aquifer that feeds streams and is also exploited to provide water supplies for much of the county and beyond. Chalk has also been used as a building material and, once fired, the resultant lime was spread on agricultural land to improve fertility. The mining of chalk since the early 18th century has left unrecorded underground galleries that occasionally collapse unexpectedly and endanger buildings.