Shedloads v. Shitloads

I always thought the ‘shedload’ was a strange unit of measurement. I can, however, imagine people using ‘shit’ to reckon amounts.

This quick Google Ngram search would suggest that this is because ‘shitloads’ came first.

Presumably, then, ‘shedloads’ is a euphemising version, or some kind of genteelism?

If anyone has any more information, please let me know.