Practising science: Reading the rocks and ecology
Practising science: Reading the rocks and ecology

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

Practising science: Reading the rocks and ecology

1.4.3 Sedimentary strata

We've seen that the detective work of piecing together a part of Earth's history from sedimentary rocks involves detailed investigation of rock samples, but this can give only a partial picture. On the larger scale of a rock exposure, there can be plenty for us to see and to interpret. Sedimentary rocks are usually found as layers referred to as strata (Figure 10), with each stratum (layer) recording the particular conditions at the time of its deposition. (Note: sedimentary layers are often referred to as beds, but strictly speaking the term ‘bed’ is reserved for those strata thicker than 1 cm; thinner layers are known as laminae (singular lamina).) Over time, conditions may have changed, either gradually or quickly, causing the nature of the sediment being deposited to change. In this way, a vertical stack of sedimentary strata is a record of changing conditions during a segment of geological time. The oldest sediments will be at the bottom, with progressively younger strata laid down on top. Geologists refer to this as the principle of superposition – older rocks are overlain by younger rocks; an individual layer is younger than the one beneath it and older than the one above it; the oldest layer lies at the bottom. This provides a relative time-scale. Changes in sedimentary rocks (or in the types of fossil they may contain) up through a sequence of strata provide a record of changing conditions over the passage of time throughout which the rocks were deposited.

Figure 10
Figure 10 A succession of sedimentary strata (layers) exposed in a cliff on the Dorset coast

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus