A planetary-scale disappearing act
In the Northwest Highlands of Scotland, you can place your hand on a line between two different rocks and touch a chasm in time. The lower, older layer is Lewisian gneiss, a gnarled, grey, metamorphic rock that is among the oldest in Europe, with parts dating back 3 billion years. Above it lie the characteristic reddish-brown layers of the Torridonian sandstone, deposited between 1.2 and 1 billion years ago. The boundary between them is the Torridonian Unconformity, a physical surface recording a missing period of geological history that spans more than a billion years.
This type of feature, where sedimentary rock like sandstone sits on top of much older metamorphic rock, is known as a nonconformity. The Lewisian gneiss was formed under intense heat and pressure many kilometers deep within the Earth's crust. The Torridonian sandstones were deposited by ancient river systems on a barren, terrestrial plain. For the two to be in direct contact, a monumental sequence of events had to occur. An entire mountain range's worth of rock had to be uplifted from deep in the crust, completely eroded away to a flat plain, and then submerged for new sediments to be laid down. The history of that uplift and erosion is the history that has been almost entirely erased.
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The billion-year gap was not an uneventful time. The formation of the Lewisian gneiss was part of the construction of ancient continental crust. Later, massive tectonic forces associated with the assembly of the supercontinent Rodinia uplifted these deep crustal rocks. This process created a vast mountain range, which was then subjected to hundreds of millions of years of relentless erosion. Wind and water gradually wore the mountains down to their roots, creating a remarkably flat plain called a peneplain. This ancient, fossilized land surface is the unconformity itself.
Around 1.2 billion years ago, sediment began to accumulate on this plain. These deposits make up what geologists call the Torridonian Supergroup, which is itself divided into older and younger sections. The oldest, the Stoer Group, contains a unique layer. One of its layers, a distinctive rock unit called the Stac Fada Member, is now identified as an ejecta blanket from a major meteorite impact that occurred in the region. This deposit contains shocked quartz and other evidence of a hypervelocity impact. The younger Torridon Group, which is about 1 billion years old, consists of sandstones deposited by vast, braided river systems flowing across a semi-arid region.