Why do you shovel not spade?

I thought and help dad out today by shovelling a pile of stones in his car. He's been meaning to get rid of them for ages but never gets round to it. I'm sure he'll be pleased when he gets home and sees I've done all the hard work.

But it got me thinking, why is when we use a shovel we're shovelling but when we use a spade we're not spading? We say "I shovelled a huge pile of manure" but we don't say "I've spaded the garden". As for having the cat spade I wish they were clearer on that before I started.
Why do you shovel not spade?
Why do you shovel not spade?
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