This could be a very short thread, but I thought I would document it in real time allowing the advice of members to influence the next steps as necessary.
Whilst out walking the dog this morning, I heard the distinctive bubbling sound of the babbling brook. I am not sure what the obstruction is. I suspect it may be a log. The area upstream is mostly willow carr with some alder but also lies along an old railway bed so there is a variety of trees in the vivinity.

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Knowing there was a bridge just downstream, I crossed to see a large, dense foam cake trapped by the bridge.

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Unfortunately, I had neither fixative nor container with me and I wanted to check the method for collecting rather than mess it up.
Tomorrow’s update should be the collected foam.
Simon