If you’re old enough to purchase alcohol, then you’re well past the age when you need your parents or any adult to be lecturing you about cleaning up after yourself.

And yet, here we are.

Last weekend, a shameful display was left on the North Shore beach at Moraine State Park. It included discarded whiskey bottles, but also dirty diapers, toys, children’s flotation devices and other refuse strewn about the grass and sand near the edge of Lake Arthur.

A local resident who took pictures of the site Sunday evening and posted them to the “Friends of Moraine State Park” Facebook page noted that the beach’s four picnic tables were also covered in empty soda cans and water bottles.

We can only hope that the empty liquor bottles — which are prohibited at the beach — and children’s belongings didn’t come from the same group of people.

Regardless, leaving a public beach in such a grotesque state is pretty pathetic. Those responsible for the mess should be ashamed of themselves.

Two people fishing at the site took it upon themselves to clean up after those who left behind the mess. We appreciate their community-minded effort.

Dustin Drew, the park’s manager, said finding a mess at the beach and other public park areas is not uncommon after a warm summer weekend. But he added, “It’s common courtesy to clean up after yourself. That’s our expectation with all of our visitors.”

As it should be.

Honestly, leaving empty alcohol containers and soiled diapers on a recreation spot that’s used by the public is selfish, immature and disgusting.

It’s also potentially dangerous: Discarded liquor could fall into the hands of children at the site, while plastic bags and other refuse threaten the local wildlife.

Drew said that people who drink alcohol on the beach or anywhere in the park could face fines up to $200. That’s great to hear, although we hope that anyone who gets caught leaving one of these areas looking like a pigsty would be penalized as well.

Remember when, as a child, your parents told you to clean up after yourself? That still applies.

— NCD