October 30, 2023

Geology Word of the Week: O is for Outcrop

Posted by Evelyn Mervine

An outcrop of sedimentary rock on Orkney, Island, Scotland. Picture taken by me in March 2023

I attempted to revive the Geology Word of the Week earlier this year, but a few busy projects at work and also writing my first book derailed my plans. Let’s try again! I have a Geology Alphabet to finish. I’ll use some photos from my trip to Scotland earlier this year to illustrate this post. Without further ado, here’s the next word…


def. Outcrop:

An exposure of rock formation at the Earth’s surface. It is a place where rocks crop out, hence the name.


Outcrops are very important for geologists. They are places where rock formations are exposed at the surface of the Earth. In many landscapes, rock formations are hidden underneath soil and vegetation. Even in the desert, rocks can be hidden underneath sand. Thus, in may places it is challenging for geologists to understand what rock formations lie underneath our feet. Outcrops are key locations where geologists can directly observe, sample, and measure rock formations.

Geologic maps are often made by collecting data from a number of outcrops and then interpreting the geology in between, which is often hidden underneath “cover”, a term that can be used to refer to soil, sand, vegetation, and so on that are covering up the rocks underneath. When working on a geologic map, geologists often take structural measurements of outcropping rocks, which provides key information about the directions in which rocks are titled, folded, and so on. The structural measurements help geologists better piece together the geology that is located underneath the cover.

Note that the term “cover” can also refer to other rock formations. Sometimes, geologists look for exposures of particular rock formations (for example, one which contains a rare ore), which may be hidden by cover that consists of other rocks (rather than soil, etc.).

Importantly, outcrops should consist of in situ rocks. That is, they should consist of rocks that are intact and which not have been transported by weathering processes. Sometimes, it can be difficult to tell at first glance if an exposed rock is an outcrop or if it is something that geologists call a “float”. A “float” is a rock or rock formation fragment that has been moved from its original location, for example by breaking off (through physical weathering) and falling down the side of a hill or, as another example, by water transport in a river or stream. In particular, it is easy to mistake large float rocks for outcrops, so it is important for geologists to take a close look and make sure they are inspecting an outcrop rather than a float. This is especially important for structural measurements, which will be incorrect for a piece of float compared to an actual outcrop.

Outcrops can be found in many locations. One great place to look for an outcrop is a cliff. Often, rock formations are exposed at cliffs. Thus, they are great places to see rocks that may be under cover elsewhere in the landscape. However, be careful when looking at rocks at a cliff. Rockfalls are common at cliffs, so you need to be cautious and take a look around for loose rocks that could fall. It may be wise to wear a helmet when studying geology near a cliff. If you are at the top of the cliff, you also need to be very careful not to fall down the cliff, however tempting it may be to get a better look at the rocks over the side!

An outcrop of rocks exposed in a cliff, Isle of Skye, Scotland. Picture taken by me in April 2023.

Cliff outcrops look similar to another place where geologists often look for rocks: a roadcut, which is an artificial exposure of a rock formation that is created when rocks are blasted away to create a path for a road. Roadcuts are great places to study geology, although they are not true outcrops. It is important to remember that roadcuts will have artificial shapes and can have blasting and drilling marks, so you need to apply a little caution when interpreting geology at roadcuts. Of course, you also need to be careful not to be hit by cars when studying rocks at a roadcut!

Another good place to hunt for outcropping rocks is in the bed of a stream or river, where erosion by water can expose rocks underneath soil and vegetation. Erosion by ocean waves can also create outcrops of rock close to the sea.

Outcrops of sedimentary rock exposed in cliffs formed by water erosion, from a stream and also ocean waves, Orkney Island, Scotland. Photo taken by me in March 2023.

Another picture of sedimentary rocks exposed in outcrop near the sea, Orkney Island, Scotland. Picture taken by me in March 2023.

Outcrops are also often found at the tops of hills or mountains, where resistant rocks can form rounded or jagged peaks. It can be more difficult for soil and vegetation to coat the tops of hills or mountains, due to factors such as steep terrain, wind, and altitude.

Rocks exposed in outcrop at the tops of hills, Isle of Skye, Scotland. Picture taken by me in April 2023.

Outcrops are great places to get a close-up look at various geological features. For example, on one of the Orkney Island outcrops I was able to observe some neat mud cracks:


Mud cracks in an outcrop of sedimentary rock, Orkney Island, Scotland. Picture taken by me in March 2023.

However, geologists do need to be careful when using outcrops to interpret what rocks are present under our feet. Harder, more durable rocks that are more resistant to erosion are more likely to be exposed in outcrop, while softer rocks may be more difficult to find in outcrop. Similarly, certain rock types, such as basalt and limestone, are more likely to form cliffs than other types of rocks. Thus, it is important not to overinterpret the abundance of rocks exposed in outcrop. To better understand the geology that lies under our feet, geologists must also use other methods, such as drilling to bring up cores of rock and deployment of geophysical instruments, together with study of rocks in outcrops.

That’s all for this week’s geology word. Stay tuned for next week!