The Highland Border Complex is an assemblage of rocks of probable early

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

to late

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

age, found as fault-bounded blocks of variable

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

exposed immediately to the southeast of the

Highland Boundary Fault The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two different geological terranes which give rise to two distinct physiographic ter ...

at the edge of the Grampian Highlands, Scotland.

Extent

Rocks of the Highland Border Complex are exposed intermittently along the trace of the Highland Boundary Fault for about 250 km from

Glen Sannox Glen Sannox may refer to: * A glen on the Isle of Arran, running from Goat Fell to the village of Sannox Sannox () is a village on the Isle of Arran, Scotland. The village is within the parish of Kilbride. The name comes from the name the Vikin ...

on the

Isle of Arran The Isle of Arran (; ) or simply Arran is an island off the west coast of Scotland. It is the largest island in the Firth of Clyde and the seventh-largest Scottish island, at . Counties of Scotland, Historically part of Buteshire, it is in the ...

in the southwest to Garron Point near

Stonehaven Stonehaven ( ) is a town on the northeast coast of Scotland, south of Aberdeen. It had a population of 11,177 at th2022 Census Stonehaven was formerly the county town of Kincardineshire, succeeding the now abandoned town of Kincardine, Aberd ...

in the northeast. The exposed width varies along the outcrop but is always less than 1,300 m. It has been suggested that the complex directly correlates with the Clew Bay Complex in Ireland, which is probably of similar age and is also associated with a proposed continuation of the Highland Boundary Fault.

Stratigraphy

At least four rock assemblages have been recognised within the Highland Border Complex: gritty sandstone with limestone and mudstone (probably lowermost

Upper Ordovician The Late Ordovician is the third and final epoch of the Ordovician period, lasting million years and spanning from around 458.2 to 443.1 million years ago. The rocks associated with this epoch are referred to as the Upper Ordovician Series. At ...

);

metabasalt Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose (fla ...

chert Chert () is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz, the mineral form of silicon dioxide (SiO2). Chert is characteristically of biological origin, but may also occur inorganically as a prec ...

and mudstone equivalents; limestone and

serpentinite Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...

conglomerate (uppermost Lower Ordovician); serpentinite, metagabbro and

hornblende Hornblende is a complex silicate minerals#Inosilicates, inosilicate series of minerals. It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole. Hornblende minerals are common ...

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

. Their relationship with the lower Cambrian Leny Limestone and associated black

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s remains debated.

Interpretation

Most of the sedimentary rocks of the Highland Border Complex have been interpreted as being in stratigraphic continuity with the Dalradian Supergroup, with the

ophiolitic An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found in the name of o ...

rocks (serpentinite, metagabbro, hornblende schist, metabasalt, chert and black slates, collectively known as the

Highland Border Ophiolite The Highland Border Ophiolite (HBO) is a set of rocks that are ophiolitic in character found within the Highland Border Complex. They are exposed in a series of fault-bounded outcrops along the line of the Highland Boundary Fault that forms the sou ...

), emplaced on them during the Grampian Orogeny. Alternatively, the complex is viewed as a tectonically emplaced sequence that is unrelated to the Dalradian.

References

{{Reflist Geology of Scotland Highland Boundary Fault