The most striking feature about the topography of Caithness is its flatness.
In Scotland, only Buchan rivals the plain of Caithness for its big skies and
distant horizons. Given its position on the fringes of the Northern Highlands,
the low relief of Caithness requires some explanation.
The plain is partly structurally-controlled. The low dips of the Devonian
flagstones lead to the development of gentle slopes. On the main hills, such as
outline of the hill is stepped and the hill top is flat, a reflection of glacial
erosion of horizontally-bedded flagstones and sandstones of uneven resistance.
Although glacial erosion has undoubtedly been significant in deepening valleys
and streamlining ridges to give a strong lineation to the relief of the plain,
it has not been sufficient to dissect or roughen the surface so that its
integrity is lost.
The plain is a major erosion surface,
created by prolonged denudation before the Ice Age. Godard (1965) pointed out
that the surface locally, as around Camster, cuts the 5-10º dip
of the Devonian and that it extends onto the basement rocks so that it not a
simple structural level
The termination of the plain in cliffs at the coast indicates that the plain has
been uplifted by around 100 m but the amount of tilting, warping and
differential movement has been modest. Godard (1965) regarded the plain as part
of his Niveau Pliocene, a level developed
around the fringes of the Highlands during the last 10 million years or so.
Across the Moray Firth, the equivalent Buchan Surface carries sediments and
weathering covers that indicate relief development during this period but the
flattening of the terrain there is partly an inherited feature. Buchan was
already an area of low relief in the Miocene and the not greatly affected by the
mighty earth movements that threw up the Scottish Highlands around 60 million
years ago. Indeed, Buchan retains traces of Cretaceous flint and greensand that
suggest that the area was of low relief before it was buried by Mesozoic
sediments. A similar history probably affected Caithness, given its similar
location adjacent to the thick Mesozoic rocks on the floor of the Moray Firth.
The plain of Caithness may therefore in outline be a very ancient feature