Cities of this kind are like hummocks of spinifex , or porcupine grass , whose centre eventually collapses , leaving live rings surrounding a dead middle .

这类城市就像是一丛丛的滨刺草或者箭猪草一样，中心已经枯死而留下一圈活体环绕着死去的中心部分。

The terrain of chongming island , which belongs to estuarine alluvial plain type , is open , low and flat , without hummocks and hills .

崇明岛地势坦荡低平,属河口冲积平原地貌类型,岛上无山岗丘陵.