Inliers and outliers (geology)
Encyclopedia
An inlier is an area of older rocks
surrounded by younger rocks. Inliers are typically formed by the erosion
of overlying younger rocks to reveal a limited exposure
of the older underlying rocks. Faulting or folding
may also contribute to the observed outcrop
pattern. A classic example from Great Britain
is that of the inlier of folded Ordovician
and Silurian
rocks at Horton in Ribblesdale
in North Yorkshire
which are surrounded by the younger flat-lying Carboniferous Limestone
. The location has long been visited by geology
students. Another example from South Wales
is the Usk Inlier
in Monmouthshire
where Silurian age rocks are upfolded amidst Old Red Sandstone
rocks of Devonian
age.
A similar outcrop pattern which results from movement on a thrust fault
followed by erosion may be termed a window
.
Conversely an outlier is an area of younger rock completely surrounded by older rocks. An outlier is typically formed when sufficient erosion of surrounding rocks has taken place to sever the younger rock's original continuity with a larger mass of the same younger rocks nearby.
One example from Great Britain is the mass of Triassic
sandstone
around the Staffordshire
town of Leek
. This is isolated from the very much larger area of Triassic rocks which characterise the English Midlands
and Cheshire Basin
, to the south and west respectively, by the surrounding Carboniferous
sandstones and mudstone
s. Similarly in the Black Mountains
of South Wales
, the summit area of Pen Cerrig-calch
is composed from a suite of Carboniferous age sandstones and limestone
. This outcrop is isolated from the main extent of these rocks (which form the South Wales Coalfield
and its margins some miles to the south) by the Devonian
Old Red Sandstone
on which it rests by and by which it is surrounded.
A similar isolated arrangement of rocks produced for example by movement on a thrust fault followed by erosion may be termed a klippe
.
Rock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
surrounded by younger rocks. Inliers are typically formed by the erosion
Erosion
Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
of overlying younger rocks to reveal a limited exposure
Outcrop
An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...
of the older underlying rocks. Faulting or folding
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
may also contribute to the observed outcrop
Outcrop
An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...
pattern. A classic example from Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
is that of the inlier of folded Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
rocks at Horton in Ribblesdale
Horton in Ribblesdale
Horton in Ribblesdale is a small village and civil parish in the Craven district of North Yorkshire, England. It is situated in Ribblesdale on the Settle–Carlisle Railway to the west of Pen-y-ghent....
in North Yorkshire
North Yorkshire
North Yorkshire is a non-metropolitan or shire county located in the Yorkshire and the Humber region of England, and a ceremonial county primarily in that region but partly in North East England. Created in 1974 by the Local Government Act 1972 it covers an area of , making it the largest...
which are surrounded by the younger flat-lying Carboniferous Limestone
Carboniferous limestone
Carboniferous Limestone is a term used to describe a variety of different types of limestone occurring widely across Great Britain and Ireland which were deposited during the Dinantian epoch of the Carboniferous period. They were formed between 363 and 325 million years ago...
. The location has long been visited by geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
students. Another example from South Wales
South Wales
South Wales is an area of Wales bordered by England and the Bristol Channel to the east and south, and Mid Wales and West Wales to the north and west. The most densely populated region in the south-west of the United Kingdom, it is home to around 2.1 million people and includes the capital city of...
is the Usk Inlier
Usk Inlier
The Usk Inlier is a domed outcrop of rock strata of Silurian age in Monmouthshire in south-eastern Wales. It is located in the countryside between the towns of Caerleon and Pontypool and the village of Raglan. The longer axis of the dome or 'pericline', often referred to as the Usk Anticline, is...
in Monmouthshire
Monmouthshire
Monmouthshire is a county in south east Wales. The name derives from the historic county of Monmouthshire which covered a much larger area. The largest town is Abergavenny. There are many castles in Monmouthshire .-Historic county:...
where Silurian age rocks are upfolded amidst Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
rocks of Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
age.
A similar outcrop pattern which results from movement on a thrust fault
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
followed by erosion may be termed a window
Window (geology)
thumb|right|350px|Schematic overview of a thrust system. The [[fault |hanging wall block]] is called a [[nappe]]. If an [[erosion]]al hole is created in the nappe that is called a window. A [[klippe]] is a solitary outcrop of the nappe in the middle of autochthonous material.A tectonic window...
.
Conversely an outlier is an area of younger rock completely surrounded by older rocks. An outlier is typically formed when sufficient erosion of surrounding rocks has taken place to sever the younger rock's original continuity with a larger mass of the same younger rocks nearby.
One example from Great Britain is the mass of Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...
around the Staffordshire
Staffordshire
Staffordshire is a landlocked county in the West Midlands region of England. For Eurostat purposes, the county is a NUTS 3 region and is one of four counties or unitary districts that comprise the "Shropshire and Staffordshire" NUTS 2 region. Part of the National Forest lies within its borders...
town of Leek
Leek
The leek, Allium ampeloprasum var. porrum , also sometimes known as Allium porrum, is a vegetable which belongs, along with the onion and garlic, to family Amaryllidaceae, subfamily Allioideae...
. This is isolated from the very much larger area of Triassic rocks which characterise the English Midlands
English Midlands
The Midlands, or the English Midlands, is the traditional name for the area comprising central England that broadly corresponds to the early medieval Kingdom of Mercia. It borders Southern England, Northern England, East Anglia and Wales. Its largest city is Birmingham, and it was an important...
and Cheshire Basin
Cheshire Basin
The Cheshire Basin is a late Palaeozoic and Mesozoic sedimentary basin which extends under most of the county of Cheshire in northwest England. It extends northwards into the Manchester area and south into Shropshire...
, to the south and west respectively, by the surrounding Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
sandstones and mudstone
Mudstone
Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...
s. Similarly in the Black Mountains
Black Mountains, Wales
The Black Mountains are a group of hills spread across parts of Powys and Monmouthshire in southeast Wales, and extending across the national border into Herefordshire, England. They are the easternmost of the four ranges of hills that comprise the Brecon Beacons National Park, and are frequently...
of South Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
, the summit area of Pen Cerrig-calch
Pen Cerrig-Calch
Pen Cerrig-calch is a subsidiary summit of Waun Fach in the Black Mountains in the Brecon Beacons National Park in southern Powys, Wales. Its summit, at a height of 701m , is marked by a trig point...
is composed from a suite of Carboniferous age sandstones and limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
. This outcrop is isolated from the main extent of these rocks (which form the South Wales Coalfield
South Wales Coalfield
The South Wales Coalfield is a large region of south Wales that is rich with coal deposits, especially the South Wales Valleys.-The coalfield area:...
and its margins some miles to the south) by the Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
on which it rests by and by which it is surrounded.
A similar isolated arrangement of rocks produced for example by movement on a thrust fault followed by erosion may be termed a klippe
Klippe
thumb|right|350px|Schematic overview of a thrust system. The shaded material is called a [[nappe]]. The erosional hole is called a [[window |window or fenster]]. The klippe is the isolated block of the nappe overlying autochthonous material....
.