The geology of the Highlands is most interesting and covers a broad spectrum from very ancient Lewisian gneisses of around three billion years of age, to much more recent sedimentary layers. The underlying rock strata continues the south-west to north-east orientation and consists primarily of rocks of the upper Proterozoic period around 500–1,000 million years of age. However, in the upper north west of the mainland and in the Isle of Lewis and throughout the Hebrides, there are significant swathes of rocks from the lower Proterozoic and Archaean periods between 1,500 and 3,000 million years of age. There are also regions of igneous and volcanic rocks to be found both on the mainland and western isles, reflecting historic volcanic activity throughout the west of Scotland. The north west of the Highlands additionally feature a thin band of rocks from the later Cambrian period around 495–545 million years ago, and there are more recent sedimentary layers to be found throughout the isles and on the mainland.