I was walking by the River Calder, near the Hepworth Gallery, when I spotted a mass of fungi on a pile of woodchips.

Wrinkled Fieldcap (Agrocybe rivulosa)
I did my best to identify them using books but again needed the help of the British Mycological Society and I found that they are Agrocybe rivulosa.

Wrinkled Fieldcap (Agrocybe rivulosa)
This species arrived in the UK just over a decade ago and has probably been encouraged to spread by the increasing use of bark and woodchip mulches. It is now common and widespread in Britain.

Wrinkled Fieldcap (Agrocybe rivulosa)
A common name used for this species is wrinkled fieldcap.