A Guide to the Geological Formations Around You

Everywhere we go we are surrounded by the story of the . We rarely think about the candy-stripped rocks or the weird outcrop in the woods that looks like a face or if the ground we walk on every day used to be the floor of an ancient sea. There are clues are all around us that hint of volcanos long gone, faults that push and pull the rocks, waterways that grind out shapes and valleys and upheavals that happened long before humans came down from the trees. Every single hill, valley, rock and lake you see has a blueprint for how it got that way. There is so much to the ’s history written in rocks, this guide barely scrapes the surface. It only provides a basic primer… an overview to naming and understanding all the different rock formations we live in and around every day.

Batholith

A batholith is a large mass of igneous intrusive rock that forms from cooled magma deep in the Earth’s crust. Batholiths are almost always made mostly of felsic, granite, quartz monzonite, or diorite. They are composed of multiple masses, or plutons, enormous bodies (picture a balloon shape) of igneous rock of irregular dimensions that can be told apart from nearby igneous rock by a combo of age and composition.

The entire Sierra Nevada range in California is a partially submerged batholith — you can see it exposed in places like Half Dome, pictured above.

Mesa

Mesas form from the weathering and erosion of horizontally layered rocks that have been uplifted by tectonic activity. Variations in the ability of different types of rock to resist weathering and erosion cause the weaker types of rocks to be eroded away, leaving the more resistant types of rocks higher than their surroundings.

Lava flows and sills, in particular, are very resistant to weathering and erosion, and often form the flat top, or caprock, of a mesa. The less resistant rock layers are mainly made up of shale, a softer rock that weathers and erodes more easily.

U-Shaped Valley

These valleys are formed by the process of glaciation. They have a characteristic U shape, with steep, straight sides and a flat bottom. Glaciated valleys are formed when a glacier travels across and down a slope, carving out the valley by the action of scouring.

When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice.

Folds

There are many types of folds. A fold occurs when one or a stack of originally flat and planar layers, such as sedimentary strata, are bent or curved as a result of permanent deformation. This is often caused by the movement of tectonic plates against each other as well as intense metamorphic pressures. These folds vary in size from microscopic crinkles to mountain-sized and they occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales.

It is super cool to look at the folds pictured above and remind yourself that each of those layers was originally flat ground.

Anticline: linear, strata normally dip away from axial center, oldest strata in center.

Syncline: linear, strata normally dip toward axial center, youngest strata in center.

Antiform: linear, strata dip away from axial center, age unknown, or inverted.

Synform: linear, strata dip toward axial centre, age unknown, or inverted.

Dome: nonlinear, strata dip away from center in all directions, oldest strata in center.

Basin: nonlinear, strata dip toward center in all directions, youngest strata in center.

Monocline: linear, strata dip in one direction between horizontal layers on each side.

Chevron: angular fold with straight limbs and small hinges

Recumbent: linear, fold axial plane oriented at low angle resulting in overturned strata in one limb of the fold.

Slump: typically monoclinal, result of differential compaction or dissolution during sedimentation and lithification.

Ptygmatic: Folds are chaotic, random and disconnected. Typical of sedimentary slump folding, migmatites and decollement detachment zones.

Parasitic: short wavelength folds formed within a larger wavelength fold structure – normally associated with differences in bed thickness.

Disharmonic: Folds in adjacent layers with different wavelengths and shapes.

Laccolith

A laccolith is a sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally flat base.

The surface rock above laccoliths often erodes away completely, leaving the core mound of igneous rock.

Volcanic Plug

A volcanic plug (also called a volcanic neck or lava neck) is a volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of pressure if rising volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption.

Glacial erosion can lead to exposure of the plug on one side, while a long slope of material remains on the opposite side. Such landforms are called crag and tail. If a plug is preserved (as you can see in the photo above), erosion may remove the surrounding rock while the erosion-resistant plug remains, producing a distinctive upstanding landform.

Scarp

There are generally two ways to get a scarp: by erosion of sedimentary rocks, or by vertical movement of the Earth’s crust along a geologic fault. At its most basic, an escarpment is a transition from one series of sedimentary rocks to another series of a different age and composition.

Escarpments are also frequently formed by faults. When a fault displaces the ground surface so that one side is higher than the other, a fault scarp is created. This can occur in dip-slip faults, or when a strike-slip fault brings a piece of high ground adjacent to an area of lower ground.

Dike

A dike is a sheet of rock that formed inside a fracture in a pre-existing rock body. (This is unlike when the new rock forms within and parallel to the bedding of a layers rock, where it is called a sill.) It is a type of intrusion, that either cuts across layers, or into a layer or unlayered mass of rock. 

Dikes can be either intrusive or sedimentary in origin. For example, when molten rock intrudes into a crack then crystallizes, it is an igneous dike. When sediment fills a pre-existing crack, it is a clastic dike.

Sill

A sill is an intrusion between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that, unlike dikes, the sill does not cut across preexisting rocks. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source.

Sills can be confused with solidified lava flows; however, there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock. On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed.

Lava flows will also typically show evidence of weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.

Rift Valley

A rift valley is a linear-shaped lowland between several highlands or mountain ranges created by the action of a geologic rift or fault. A rift valley is formed on a divergent plate boundary, the boundaries between two tectonic plates that are pulling apart from each other. When the tensional forces were strong enough to cause the plate to split apart, a center block drops down between the two blocks at its flanks, forming a graben. This drop of the center creates the nearly parallel steeply dipping walls of a rift valley when it is new.

This feature is the beginning of the rift valley, but as the process continues, the valley widens, until it becomes a large basin, that fills with sediment from the rift walls and the surrounding area. Rifts can occur at all elevations, from the sea floor to plateaus and mountain ranges. They can also occur in continental crust or in oceanic crust.

Hogback

A hogback is a long narrow ridge or series of hills with a narrow crest and steep slopes of nearly equal inclination on both flanks. The name for this landform refers to its resemblance in outline to the back of a hog.

It was named after a hogback, Hog’s Back, of the North Downs in Surrey, England. This term is also used for any ridge with a sharp summit and steep slopes of nearly equal inclination on both flanks.

Basalt Columns

Predominantly hexagonal in cross-section, these mathematical-looking columns form during the cooling of a thick lava flow when contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it can’t easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns.

The size of the columns depends on how rapidly the cooling of the lava was. Very fast cooling makes smaller (1cm in diameter) columns, while slower cooler gives us the larger variety.

Exfoliation

Exfoliation is an erosional process that consists of a process of separation of successive thin shells, or spalls, from massive rock such as granite or basalt; it is common in regions that have moderate rainfall. The thickness of individual sheet or plate may be from a few millimetres to a few metres.

A small-scale form of exfoliation, called spheroidal weathering, is restricted to boulder-sized rock material and may occur at some depth within the Earth. In this case, rounded boulders are found surrounded by layers of disintegrated material.

Scree/Talus

Scree is a collection of broken rock fragments at the base of crags, mountain cliffs, volcanoes or valley shoulders that has accumulated through rockfall from adjacent cliff faces. Scree formation is commonly attributed to the formation of ice within mountain rock slopes. During the day, water can flow in joints and discontinuities in the rock wall.

If the temperature drops sufficiently, this water may freeze. Freeze-thaw scree production is thought to be most common during the spring and fall, when the daily temperatures fluctuate around the freezing point of water, and snow melt produces ample free water.

Thrust Fault

A thrust fault is a type of fault with 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.

Frost Heave

These ‘lenses’ are formed by an upwards swelling of soil during freezing conditions caused by an increasing presence of ice beneath as it grows towards the surface. The weight of overlying soil restrains vertical growth of the ice and causes the “lens-shaped” areas of ice within the soil.  

The soil that covers the ice lenses must be sufficiently porous to allow capillary action, yet not so porous as to break capillary continuity.

Abrasion

This is the sort of weathering that often occurs to rocks in rivers, beaches, or desert areas. Water and wind can easily remove the upper layer of a rock with friction. The rock will not only reduce in size, it will also become invariably spherical or oblong in shape and extremely smooth. Other forces that may cause friction besides waves and wind include glacier movements, gravity, or erosion.

These will all weather the rock. Besides rocks, entire shores can suffer from coastal abrasion. Breaking waves that contain sand and other fragments from sea cliffs will carve out the land itself.  An abrasion platform is a shore in which waves constantly cause friction and reduction of the land.

Stack

A stack (or sea stack) consists of a steep and often vertical column or columns of rock in the sea near a coast, formed by erosion. They are formed when part of a headland is eroded by the force of the sea or water crashing against the rock.

The force of the water weakens cracks in the headland, causing them to later collapse, forming free-standing stacks and even small islands.

Spit

Spits are created by deposition of sediment. It is an extended stretch of beach material that projects out to sea and is joined to the mainland at one end. They form where prevailing winds blow at an angle to the coastline, resulting in longshore drift.

Block Weathering

Block weathering occurs when solid rock outcrops fracture. The most important of mechanical weathering forces is temperature, which fluctuates above and below freezing almost every day. Water in cracks expands as it freezes at night, forcing cracks wider, then trickles further into the expanded crack during the day. The daily cycle of temperature also affects the different minerals in the rock, which expand and contract at different rates and cause the grains to loosen. 

As blocks work their way loose and form steep deposits of talus, their edges begin to wear down and they officially become boulders. When erosion wears them down smaller than 256 millimeters across, they become classified as cobbles.

Tafoni

Also known as cavernous weathering, this is a type of mechanical weathering where you find small cave-like features in granular rock such as sandstone, granite, and sandy-limestone with rounded entrances and smooth concave walls, often connected, adjacent, and/or networked.

Currently favored explanations controlling their formation include salt weathering, differential cementation, structural variation in permeability, wetting-drying, and freezing-thawing cycles, variability in lithology, case hardening and core softening, and/or micro-climate changes and variation. Tafoni have also been called fretting, stonelace, stone lattice, honeycomb weathering, and alveolar weathering.

Avulsion

Avulsion is the abandonment of a river channel and the formation of a new river channel. This usually occurs as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course.

There are 4 types of avulsion: erosional (when the banks of the course simply erode away), deltaic(pictured above), meanders (when a river cuts off a sharp curve of itself in favor of a shorter path), occurence (usually a violent change to the environment, like a flood).

Monadnock

A monadnock is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. Volcanic or other processes may give rise to a body of rock resistant to erosion, inside a body of softer rock such as limestone which is more susceptible to erosion.

When the less resistant rock is eroded away, the more resistant rock is left behind as an isolated mountain. The strength of the uneroded rock is often attributed to the tightness of its jointing.

Tor

Tors are another type of landform created by the erosion and weathering of granitic rocks, typically schists, dacites, dolerites, and coarse sandstones. They can be monolithic, but are more usually subdivided into stacks, often arranged in avenues.

Each stack can comprise several tiers or pillows, which may become separated: rocking pillows are called logan stones. These stacks are vulnerable to frost action, often collapsing, with trails of blocks called clitter down the slopes.

Stone Run

A stone run (called also stone river, stone stream or stone sea) is a rock landform that is the result of the erosion of particular rock varieties caused by the freezing-thawing cycles that took place in during the last Ice Age. The actual formation of stone runs involved no less than five processes:weathering, solifluction, frost heaving, frost sorting, and washing.

These stone runs are essentially different from moraines, rock glaciers, and rock flows or other rock phenomena involving the actual flow of rock blocks under stress and are instead fixed stably. The accumulations of boulders have no finer material between them.

Alluvium

Alluvium is loose soil or sediment which has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose material is deposited or cemented, it is called an alluvial deposit or alluvial fan.

It is usually deposited in the lower part of the course of a river, forming floodplains and deltas, but may be deposited at any point where the river overflows its banks or where the velocity of a river is checked—for example, where it runs into a lake.

Desert Pavement

Desert pavement is a desert surface covered with closely packed, interlocking angular or rounded rock fragments of pebble and cobble size. They typically top alluvial fans. The most common formation theory suggests that the gradual removal of sand, dust and other fine-grained material by the wind and intermittent rain leaves the larger fragments behind which are shaken into place through the forces of rain, running water, wind, gravity, creep, thermal expansion and contraction, wetting and drying, frost heaving, animal traffic, and the Earth’s constant microseismic vibrations.

The removal of small particles by wind does not continue indefinitely, because once the pavement forms, it acts as a barrier to resist further erosion.

Oxbow Lake

An oxbow lake is a U-shaped body of water that forms when a wide meander from the main stem of a river is cut off, creating a free-standing body of water. This landform is so named for its distinctive curved shape, resembling the bow pin of an oxbow. This is one of the results of a type of river avulsion.

Bioturbation

This formation is the reworking of soils and sediments by animals or plants. Common bioturbators include annelids (“ringed worms”); bivalves like mussels,clams, gastropods, holothurians; or any other infaunal or epifaunal organisms. 

Concretions

Concretions are pseudofossils bound with mineral cement in curious shapes. When minerals in water are deposited about any type of nucleus (such as a leaf or shell or other particle) they form a rounded mass whose composition is usually different from the surrounding rock.

They are often spherical in shape, although irregular shapes can occur.

Laminations

Lamination is a small scale sequence of fine layers of sediment in rock. Laminations are normally smaller and less pronounced than bedding layering, though weathering can enhance the differences between layers. A single sedimentary rock can have both laminae and beds, the bedding being defined as thicker than one centimeter and the laminae as being less. 

They are caused by changes in the supply of sediment that affect grain size, color and composition.

Amygdules

These are mineral-filled bubbles in petrified lava. They form when the gas bubbles or vesicles in extrusive igneous rocks are filled with another mineral like calcite, quartz or chlorite.  Some amygdules are stretched, not round.

These started out as spherical vesicles and were deformed while the lava was still fluid. Amygdules and vesicles are usually indicators of movement in ancient lavas.

Fiamme

Fiamme are lens-shaped, usually millimetres to centimetres in size, seen on surfaces of some volcaniclastic rocks. Some fiamme represent fragments of volcanic ejecta that have been flattened by compaction and/or shear.

Vein

A vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by water within the rock mass are deposited. They are usually thought of as being the result of growth of crystals on the walls of fractures in rocks, with the crystal growth happening on the walls of the cavity, and the crystal protruding into open space.

This certainly is the method for the formation of some veins. However, it is rare in geology for significant open space to remain open in large volumes of rock, especially a long distance below the surface. Thus, there are two main mechanisms considered likely for the formation of veins: open-space filling and crack-seal growth.

🍀ViraLuck