The South Wales Valleys hosted Britain's only mountainous coalfields. Topography defined the shape of the mining communities, with a "hand and fingers" pattern of urban development. There were fewer than 1,000 people in the Rhondda valley in 1851, 17,000 by 1870, 114,000 by 1901 and 153,000 by 1911; but the wider impact of urbanisation was constrained by geography—the Rhondda remained a collection of villages rather than a town. The population of the Valleys in the late 19th and early 20th centuries was disproportionately young and male; many of them were migrants drawn from other parts of Wales or from further afield. The new communities had extremely high birth rates—in 1840, more than 20 per cent of Tredegar's population was aged under seven, and Rhondda's birth rate in 1911 was 36 per thousand, levels usually associated with mid-19th century Britain.