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The three parallel lines of Glen Roy mystified geologists and other scientists for many years. Swiss geologist Louis Agassiz travelled to Scotland in 1840 to develop his theories about the work of ice and solved the puzzle. Professor Iain Stewart takes a flight over Glen Roy in a microlight to show how evidence from Scotland supported Agassiz' theories. It is now accepted that a glacier blocked the Glen and its river. Rising water formed a lake which created a level shore line. A colder climate caused the glacier to increase in height, blocked the valley further up and the level of the lake rose, creating a new, higher shore line. When the ice melted the shore lines of the glacial lake remained, high above the valley floor.