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.