Igneous intrusions follow lines of weakness often related to regional tectonic stresses. While the Cuillin mountains were still active volcanoes North America and Europe were being torn apart by the opening of the North Atlantic. The crust was being stretched in and E-W direction, producing N-S oriented dykes. Here we see several dykes which have been injected along the same line of weakness, with each successive dyke cutting through the middle of the previous intrusion. This results in the odd phenomenon of dykes which apparently have chilled margins on only one side as they have been split in half by later intrusions. This is how the ocean floor is produced at spreading centres such as the mid-Atlantic ridge, which is still producing fresh seafloor at a rate of around 5 cm per year on average. The parallel dyke systems, with their asymmetric chilled margins found on the ocean floor are called sheeted dyke complexes. You can use the asymmetry of chilled margin expression to tell in which direction the spreading centre was if you find sheeted dykes preserved in segments of ancient sea floor, which are sometimes preserved as ophiolites during mountain building events.

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