Overall, the Hunter Valley has more soils (mostly hard, acidic patches of poorly draining heavy clay) that are unsuitable for viticulture than they have areas that are ideal for growing grapes. The soils of the Lower Hunter vary widely from sandy alluvial flats (often planted to Semillon), to deep friable loam (often planted with Shiraz) and friable red duplex soils. In the Upper Hunter, the rivers and creeks of the region contribute to the areas black, silty loam soils that are often overlaid on top of alkaline clay loam. Among the hills of the Brokenback range are strips of volcanic basalt that are prized by growers for their tendencies to restrict vigor and concentrate mineral flavours in the grapes.