There was an orgy in the Chesapeake Bay last weekend, and intrepid boatman was on hand to chronicle it.

It occurred off the mouth of the Potomac River in the big water called the Southwest Middle Grounds. Here, the Bay floor rises up shallow after you cross the deep ship channel that runs from Norfolk to Baltimore and beyond.

The principals, observed in public embrace and advancing from every point on the compass over the slick-calm water, were crabs; doubler crabs, to be precise, courting and on the verge of mating.

Where they were headed is hard to say, but the intent was clear. These crabs were in a reproductive mood.

Crabs mate when the females are in the middle of a molt. As everyone knows, they grow by shedding their old hard shells when they get too fat and then growing new, larger shells.

For a brief period just after the molt, the crabs are soft, pliable and vulnerable to all predators, including humans who prize the tasty softshells. The females can mate only during this stage.

During the hours of vulnerability they have protectors. Male ("jimmy") crabs sensing the impending molt of a female will vie with one another to squire her through the delicate process. Whosoever is chosen becomes her protector while she sheds her shell and grows a new one. In return, the jimmy wins the right to sire her young. It's the old squire-and-sire trick.

The doublers we saw skimming the smooth surface of the Chesapeake were jimmies cradling females in their nasty little sex-crazed arms, preparing for the molt.

We were out to fish, and having a fine hot time of it, but with all those randy crabs flitting around oblivious to danger, we couldn't resist snatching a few in our nets as well, and after shaking off the little females (certain to find new mates before the magic moment), we managed to keep 18 fat jimmies.

Did we mention the Ice Age?

No? Meant to. See, the deep ship channel you cross to get to the orgy grounds is the old streambed of the Susquehanna River, which used to run down the middle of the Chesapeake before there was a Chesapeake. When the Ice Age ended, all the water caught up in the great glaciers melted and sea level rose the world around, flooding among other things the Susquehanna basin and creating the Bay, America's largest estuary and hotbed of crab cohabitation.

This was maybe 15,000 years ago, the blink of an eye in geological terms, but what a difference a blink makes!

On Sunday it was so hot in this formerly frigid riverbank that the bluefish came out of the water poached. Just drip a little lemon sauce on and chow down. Oyster toads you couldn't get off the hook -- they'd scald your hand.

We threw the crabs in a bucket and 15 minutes later they were not only steamed, they had Old Bay hot seasoning on them.

It made me think back to the good old days of the Ice Age last winter, when we were navigating five miles in the dark through the marshes of Taylor's Island Wildlife Management Area on the Eastern Shore in a blinding sleet storm after a day of less than auspicious duck hunting.

We ran the heat and the blower up on the Wagoneer when we got out and kept it on all the way home, but my mustache was still frozen when we hit Washington. They had to take me to one of those steam-clean-your- engine places and put me under the nozzle for 10 minutes just so I could eat.

Thinking about the cold old days helped relieve us as we sat in the sweltering heat 10 miles from shore, swatting at greenhead flies, watching the seagulls keeling over in a dead faint and the sea nettles pulsating in the hot salt water.

The coldest day on the water I could remember was pretty cold, but seemed even more so because I wasn't on the water -- it was on me. That would be the Susquehanna Flats, the day after Christmas, when Bobby Jobes led a band of us goose-hunting in the old "body-booting" style, breaking ice to get to the hunting grounds.

You don chest waders and squat down in three feet of water, hiding behind a giant goose decoy in a layout of 200 smaller decoys. This works great until some geese threaten to come in, at which time you're so busy calling and hiding and getting the gun ready that you forget where you are and duck down a hair too far and the icy Bay pours down your rubber pants.

Ahhhhhh. That felt good.

And typical. Leave it to a puritanical ex-New Englander to pour ice water on a nice orgy.