A Guide To The Geological Formations Around You

Everywhere we go we are surrounded by the story of the Earth. We rarely think about the candy-striped 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 Earth'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.

Photo: James St. John / Flickr / CC BY 2.0

  • 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.

  • 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
    Photo: Nathaniel H. Darton / Wikimedia Commons / Public domain


    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.