Fieldwork Skills » Roundness Index and Other Indices

What you'll learn this session

Study time: 30 minutes

What the Roundness Index is and how to calculate it
How to collect and record data for Roundness Index studies
Other important geographical indices including Sphericity Index and Flatness Index
How to apply these indices in real fieldwork situations
Practical techniques for measuring and sampling
How to interpret and present your findings

Introduction to Roundness Index

The Roundness Index is a key measurement technique in geography that helps us understand the physical characteristics of sediments and landforms. By measuring how 'round' or 'angular' stones and pebbles are, we can learn about their history, transport processes and the environments they've been through.

Key Definitions:

Roundness Index: A numerical measure of how rounded a stone or pebble is, ranging from 0 (very angular) to 1 (perfectly round).
Sediment: Fragments of rock that have been eroded, transported and deposited.
Attrition: The process where sediments become more rounded as they collide and rub against each other during transport.

📐 Why Study Roundness?

Measuring roundness helps geographers understand:

How far sediments have travelled
The energy of the transport medium (river, glacier, etc.)
The history of sediments and landforms
Different erosional processes at work

🔬 Practical Applications

Roundness studies are useful for:

River studies - examining downstream changes
Beach studies - comparing different coastal environments
Glacial studies - distinguishing between glacial and fluvial deposits
Understanding landscape evolution

Calculating the Roundness Index

The most common method for calculating the Roundness Index was developed by Cailleux. It's a simple but effective way to quantify how round a stone is.

The Cailleux Roundness Index Formula

The formula is:

Roundness Index = (2r / L) × 1000

Where:

r = radius of the sharpest corner (in mm)
L = longest axis of the pebble (in mm)
The result is multiplied by 1000 to give a more manageable number

The higher the index value, the more rounded the stone. Very angular stones might have values below 100, while well-rounded pebbles could have values over 500.

How to Measure Roundness in the Field

Follow these steps to measure the roundness of sediments:

Collect a sample of stones or pebbles (usually 20-50 per site)
Measure the longest axis (L) using a ruler or callipers
Identify the sharpest corner of the pebble
Use a curvature gauge or trace the corner onto graph paper to measure the radius (r)
Apply the formula to calculate the Roundness Index
Record your results in a table

Practical Tip: Sampling Strategy

When collecting samples, use random sampling to avoid bias. You can use a quadrat or collect every 10th pebble you encounter. Make sure your samples are of similar size (e.g., all between 3-6 cm) to ensure fair comparison.

Other Important Indices in Sediment Analysis

While the Roundness Index is valuable, geographers often use several other indices to fully understand sediment characteristics.

📑 Sphericity Index

Measures how close a pebble is to a perfect sphere. It compares the three axes of the pebble:

Formula: Sphericity = ∛(abc/L²)

Where a, b, c are the three axes and L is the longest axis.

Values range from 0 to 1, with 1 being a perfect sphere.

📏 Flatness Index

Measures how flat a pebble is relative to its other dimensions.

Formula: Flatness = (a + b) / 2c

Where a is the longest axis, b is the intermediate axis and c is the shortest axis.

Higher values indicate flatter pebbles.

📊 Powers Scale

A visual comparison method using six classes of roundness:

Very angular (0.12-0.17)
Angular (0.17-0.25)
Subangular (0.25-0.35)
Subrounded (0.35-0.49)
Rounded (0.49-0.70)
Well-rounded (0.70-1.00)

Field Methods and Data Collection

Collecting good quality data is essential for meaningful analysis. Here's how to conduct a thorough field investigation:

Setting Up Your Study

Before heading to the field, plan your investigation:

Decide on your sampling locations (e.g., different points along a river)
Prepare data collection sheets
Gather equipment: ruler, callipers, curvature gauge, camera, GPS
Decide how many samples to collect at each site

Recording Your Data

Create a table like this to record your measurements:

Sample #	Location	Longest axis (L) mm	Radius of sharpest corner (r) mm	Roundness Index (2r/L)×1000	Notes
1	Site A	65	3.2	98.5	Angular, quartz
2	Site A	48	5.1	212.5	Sub-rounded, granite

⚠ Common Mistakes to Avoid

Selecting only the most interesting or attractive pebbles (sampling bias)
Measuring too few samples to be statistically valid
Forgetting to record the exact location of each sampling site
Not considering other factors that might affect roundness (rock type, size)
Inconsistent measuring techniques between different people or sites

💡 Analysis Tips

Calculate the mean roundness value for each site
Create histograms to show the distribution of roundness values
Use scatter graphs to show relationships (e.g., roundness vs. distance downstream)
Apply statistical tests to determine if differences between sites are significant
Compare your findings with existing studies or models

Case Study Focus: River Pebble Roundness

A study of the River Tees in northern England showed that pebble roundness increased from 125 (angular) near the source to over 400 (well-rounded) at the mouth. This demonstrates how attrition during river transport gradually rounds the sediments. Interestingly, tributaries introduced new angular material, causing temporary drops in the average roundness values. This case study shows how roundness indices can reveal the complex history of sediment transport in a river system.

Interpreting Your Results

Once you've collected and calculated your roundness data, you need to interpret what it means:

High roundness values (400-600+): Indicate long transport distances or high-energy environments with lots of collisions (beaches, fast rivers)
Low roundness values (50-200): Suggest short transport distances or recent weathering from bedrock
Mixed roundness values: May indicate multiple sources of sediment or complex transport history
Patterns of change: Gradual increases in roundness along a river suggest normal attrition processes
Sudden changes: Could indicate new sediment sources, changes in gradient, or human interventions

Remember that rock type also affects roundness - softer rocks like limestone round more quickly than harder rocks like quartzite. Always note the rock type when possible.

Presenting Your Findings

Effective presentation of your data is crucial. Consider these methods:

Line graphs showing changes in average roundness between sites
Bar charts comparing different locations
Scatter plots showing relationships with other variables
Maps with pie charts showing the proportion of different roundness categories at each site
Photographs of typical samples from each site

Conclusion

The Roundness Index and other sediment indices are powerful tools for understanding geographical processes. By measuring these properties, you can uncover the hidden stories of how landscapes form and change over time. These techniques are accessible, requiring minimal equipment, but provide valuable scientific data that can help explain complex environmental processes.

In your iGCSE fieldwork, applying these indices will demonstrate your technical skills and ability to collect, analyse and interpret geographical data - all essential components of successful fieldwork investigations.

🧠 Test Your Knowledge!