Ask-a-Geologist #10: What Are The Remains Of Once Living Organisms Found In Sedimentary Rocks?

Hi Everyone! This week Shane wrote to us and asked: What are the remains of once living organisms found in sedimentary rocks?

Hi Shane, Fossils are the remains or impressions of once living organisms that you can find in sedimentary rocks. There are two basic types of fossils that geologists and paleontologists talk about. These are body fossils and trace fossils. Body fossils are a real body part of an animal such as a dinosaur bone or a shark tooth. A trace fossil is something that shows evidence that something was there such as tracks, burrows, trails, molds, casts, and impressions.

The shark teeth and snail shell that I show in this video are examples of body fossils. These types of fossils can either be the original shell, tooth or bone material or the fossil could be altered somewhat but the shape and details of the original are intact. The rocks that I show here are fossiliferous limestone. This rock gets its name from the abundant fossils that are a part of the sample. Some are bits of shells but some are trace fossil such the impression of a shell and areas which appear to be burrows from animals. You can also see a fossil that looks like a snail shell that was replaced by limestone but holds onto the shape of the original part. Other trace fossils that you might see in books or collection include leaf and fish impressions that are common in sedimentary rocks.

Remember to keep your questions coming in through the dig into geology section on our website or email us at Please comment below and subscribe to our channel so that you get the next installment of ask a geologist. Until next time, rock on everybody.



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