5
3 1 Geological formation of stacks
A study of the coastal sites of Lewis must take
into account the geological and geomorphological
processes of site formation. The topography of the
Lewis coastline is determined in part by the islands’
underlying geology. The physical structure of the
rocks at the shore determines the formation of
stacks and inlets. Although in summary the solid
geology of Lewis is relatively uniform, comprising
Lewisian gneiss with metasediments and conglom-
erates in more localised areas, in reality, the more
closely it is studied, the more complex it becomes
(Angus 1997, 8).
However, the geology is only the medium from
which the sculptured shoreline is made. The creation
of the stacks, geos (inlets) and reefs of Lewis is also
determined by other variables, including the pre-
Pleistocene landform, climate, marine variability,
sea level rise and the availability of marine and
organic sediment (Angus 1997, 84). The erosion of
the coasts by the sea is the main influence acting
on these variables. During severe winters the west
coast of Lewis will be subject to breaking waves
exerting a pressure exceeding 24 tonnes/m2 (ibid,
87).
This continual bombardment of the coastline
eventually compromises weak bedding planes and
soft rocks, leading to the formation of a variety
of different features depending on the geological
structure of the bedrock. Where weaker or softer
strata lie parallel to the shoreline across a
peninsula, sea caves will form and eventually meet,
leading to the creation of an arch, and following the
collapse of the arch, to an isolated stack. If weaker
rocks are perpendicular to the shoreline, geos and
inlets will form.
The term ‘stack’ is difficult to define, but usually
refers to an isolated pinnacle of rock entirely sur-
rounded by the sea at high tide (Mellor 2002, 2). If
its summit has a larger diameter than its height,
then it is an island. Many of the sites included in
this project are islands or promontories, despite
being described as ‘stacks’ by their place names.
For instance, Stac Domhnuill Chaim (see Section
6), and Stac a’Chaisteal (see Section 7), are both
permanently joined to the mainland. The literal
translation of the Gaelic stac given by ‘Dwelly’s dic-
tionary’ is ‘precipice, steep, high cliff or hill’ (Dwelly
1994), and usually applies to any topographic
feature which has a pinnacle-like or hay-stack-like
shape, including the mountains Stacaiseal, Lewis
(NB 3065 3740) and Stac Pollaidh (NC 108 105)
and Beinn Stac (NC 270 422) in the north-west
Highlands.
3 2 Location of stack sites in Lewis
Chris Burgess undertook a large amount of research
on promontory enclosures in Lewis throughout the
1990s (Burgess 1999, 2000); a research interest that
developed from his work surveying the coastline
of Lewis during the Coastal Erosion Assessment
(Burgess 1999, 93). He admitted that the ‘promon-
tory enclosure’ description of these sites may well be
an inaccurate one: many of the sites are not ‘enclosed’
but merely ‘barred’ (ibid), and tidal stack sites are
also included in the overall term. He does however
divide his general terminology into four sub-categor-
ies: promontories, headlands (both barred), coastal
enclosures and stacks (which may be linked to or
separated from the shore; ibid 96).
The CEAL survey classified 80 sites as promon-
tory enclosures (Burgess 1999, 94). The general
distribution of these sites covers much of the Lewis
coastline but Burgess divided them into five general
areas (ibid, 95–96).
Area 1 – Brenish, Islivig and Camas Uig (17 sites):
typified by high eroding sea cliffs with a mixture of
high and low promontories and several knife-edge
stacks.
Area 2 – Bernera and the Loch Roag complex (12
sites): typified by low eroding edges punctuated by
shingle beaches. This area includes Bernera and the
Bhaltos Peninsula.
Area 3 – Garenin to Barvas (13 sites): dominated
by high Atlantic cliffs with frequent promontories,
interspersed with small bays of sand and shingle.
Area 4 – North Galson, Ness and Skigersta (16 sites):
dominated by machair to the west and high cliffs
to the east all backed with flat croft land, this area
is the smallest of the five. The coastline has a com-
bination of extensive, low promontories and large
eroding stacks on which sites are situated.
Area 5 –Tolsta, Broad Bay and the Eye Peninsula
(14 sites): this area is the most dispersed and covers
the biggest geographic area, with high eroding sea
cliffs to the north at Tolsta, and low eroding cliffs
formed from soft conglomerate rocks in the Broad
Bay area. The Eye Peninsula also has many high
sea cliffs punctuated by stacks and promontories,
with a change further south as the terrain becomes
one of low eroding edges and gentle stacks.
Burgess noted that stacks and promontory enclos-
ures are most common in Area 4, forming 25% of the
3 THE FORMATION AND LOCATION OF STACKS IN
LEWIS
