Like most of Scotland's lochs, Loch Lomond was formed by the movement of glaciers during the last ice age 10-20,000 years ago. The narrow northern section was created by glaciers that carved a deep fissure into the hard rock north of the Highland Boundary Fault. The soft rocks south of the Highland Boundary Fault allowed glaciers to spread out, hence the reason why the southern end of Loch Lomond is much wider.