Pebble Size and Shape

Did you collect some pebble samples last week?

Hopefully you got a sample of 10 for each site and you labelled each bag so that you know which came from which site!

We are now going to see if there is a pattern to how the pebbles change from upstream to downstream along the river.  Do the pebbles get larger or smaller?  Do they get more angular or more rounded?

Let’s start with size.  How do you measure a pebble?  The easiest way to do it is to measure the long axis.  That’s the longest length that you can find on the pebble. 

You can do it by eye on a ruler or you can get a more accurate result by lining your pebble up between 2 blocks (thick books will do unless your pebbles are huge). 

Move the blocks to just touch the furthest ends of the pebble.  (Work against a straight edge to keep everything square.)  You can then let go of the pebble and measure accurately between the 2 blocks.

Measure the long axis of all 10 pebbles from site 1.  Add the results together and divide by 10 to find the average pebble length for site 1.  Do the same for your other sites and then see if there is a pattern.  Are the pebbles downstream bigger or smaller?

Right, what about shape? 

You need to compare your pebbles with the ones in the illustration.  This is subjective, which means that you must decide your opinion on the shape, but your friend might decide a bit differently.  Hopefully you will be able to agree enough to come up with the same results overall.

For each site, sort out your pebbles into the different types of shape and see which type has the most pebbles.  Compare upstream sites and downstream sites.  Is there a change?