The Scandian Event of the Caledonian Orogeny

The second stage of the Caledonian Orogeny occurred when Baltica (Scandanavian Europe) collided with the foundations of the Northern Highlands around 440 million years ago. This collision pushed the Northern Highlands westwards onto the foundations of the Northwest Sea-board and is known as the Scandian Event. It also formed the hard metamorphic Moine rocks, originally layers of sea-floor sediment. The movement westwards occurred along zones of shearing in the rock mass (shear zones) and also along brittle fractures or faults. The shear zones can still be traced in the Moine rocks of the Northern Highlands and are sometimes referred to as slides.

The most significant brittle faults associated with the Scandian Event are those of the Moine Thrust Belt (or Moine Thrust Zone). This marks the furthest west where rocks occur that were affected by the Scandian Event. The Moine Thrust, itself marked the western extent of the Moine rocks and thus the edge of the Northern Highlands. However, as the Moine rocks were pushed over the rocks of the Northwest Sea-board along the Moine Thrust, the rocks below were dragged along with them. As a result, these rocks at the edge of the Northwest Sea-board were fractured by large faults and in some cases altered by heat and pressure. This zone of faulting below the Moine Thrust is now exposed to the west of the Moine Thrust by erosion, and is known as the Moine Thrust Belt (or Moine Thrust Zone).

The main phase to the Scandian Event, which pushed the Northern Highlands westwards, was followed by a phase of lateral movement along faults, including the Great Glen Fault and the Walls Boundary Fault in Shetland. This started the process of bringing the amalgamated Northwest Sea-board and Northern Highlands together with the Grampian Highlands. The lateral movements probably continued until around 410 million years ago, by which time the Northern Highlands and Grampian Highlands had come together.

During and after the Scandian Event, molten rock formed and pushed its way into the deformed rocks of the Northern Highlands. Masses of cooled molten rock, known as igneous intrusions, in the Northern Highlands are not as abundant as in the Grampian Highlands. The largest igneous intrusions occur around Strontian, Strath Halladale and Helmsdale, with smaller ones elsewhere including within the Moine Thrust Belt.