Teacher:In this task, you are given a paragraph in Chinese, and your task is to construct a question in Chinese from the given passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique, and (iv) the answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses the same words or phrases given in the passage. 
Teacher: Now, understand the problem? Solve this instance: 罗西里 (Rhossili) 是英国威尔斯风景区Gower半岛(Gower Peninsula)西南端的一个小村庄。在英国是有名的观光点。在这里出生的Edgar Evans在1911年跟随探险家罗伯特·斯科特(Robert Falcon Scott)到达南极，他在回程中第一个丧生。罗西里湾呈弧状，由村落开始向北伸延。罗西里湾的南端有一个叫Worm's Head（虫头）的小岛，和Gower半岛之间有天然海堤连接，潮退时可以步行前往。著名威尔斯诗人狄兰·托马斯(Dylan Thomas)曾经因为未能赶及在潮涨前离开而被困岛上一晚。（参照英文原文：http://en.wikipedia.org/wiki/Rhossili）
Student:
Worm's Head（虫头）的小岛和Gower半岛之间为什么潮退时可以步行前往？