What's happened to my lawn?

My lawn looked perfectly healthy, lush and green but was getting a bit long (approx 3 inches) so decided to give it a mow this morning. Once mowed, I've discovered clumps of mud all over my lawn! Now it looks horrific! Have a look at the photos bellow... What's happened!?


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So it should be fine? I don't need to go round removing the soil, it'll just sort itself out over time?

 

Yep, you should see mine where i've been squelching thru the thing, bare patches everywhere, but it'll come back in the spring

 

Loofah is right I think. My lawn is exactly the same. The worm casts can be raked away when dry or frozen, but when wet they collapse into mud smears just like that. Walking on it making it worse. It will be ok when it starts growing again.

 

This is just worms. I would advise spraying some verdon on the small muddy bits as they are prone for growing weeks in them