IF YOU EVER want to get noticed, try walking through the streets of Sea Cliff on a weekday afternoon. Especially when wearing old jeans and a berry-splattered long-sleeved shirt and toting a tattered canvas bag full of empty, 32-ounce Nancy's Yogurt containers over your shoulder. Hermetically sealed SUVs will glide by you, humming suspiciously. The whole place is eerily depopulated and silent; anyone can walk the sidewalks here, but no one does. But just yards away from this excruciatingly exclusive real estate, where even the roses seem Botoxed, you can see the whole gorgeous Pacific panorama for free. And, even better, you can forage.

Now, urban foraging can be a tricky proposition. Out in the woods, if you know what to look for, you can snag mouthfuls of blackberries, buckets of persimmons, fingernail-staining handfuls of walnuts fresh from the trees. In the city, foraging is often more like bartering. I've shamelessly turned chance encounters with friends of friends into on-the-spot invitations to reap the bounty of their Meyer lemon trees (the trade was a fresh lemon pound cake, delivered to their door the following day – generosity should always be rewarded in kind).

Anyone visiting me when I had my tiny community-garden plot went home with a bagful of lettuce and a bundle of sage and rosemary. Figs dropped with a ripe splatter on the cars parked behind my old apartment on Guerrero Street. I've even made lemon meringue pie from gnarly lemons growing behind El Rio.

Blackberries, however, are a truly wild pleasure. The big, tough, thorny invaders known as Himalayan blackberries are no backyard friend: their snarling vines thrive on neglect and will turn any empty garden into a rough, impenetrable wilderness of bees and berries. Like mint and ivy, two other plants that seem so picturesque from a distance, blackberry vines aim to colonize. Even the normally forgiving Golden Gate Gardening (a must-have tome for any Bay Area gardener) warns against letting blackberries take hold in your garden.

All the more reason to get out into the fresh air to find them, then. Blackberries like sandy, scrubby landscapes, often in sight of water. Any dedicated urban forager will have his or her favorite spots: around Bernal Hill, somewhere in the Presidio, or my favorite place, just above the path down to China Beach, a short walk from Sea Cliff. As you walk down to the cove, there are pine trees and all manner of scruffy little shrubs. Look closer, though, and you will see, wound among and below the pine trees, thick, thorny vines. Sniff and, over the scents of pine and brine, you'll catch a deep, winy fragrance of sun-ripe berries.

Wear thick shoes and long pants, and be prepared to get scratched. More important, make sure to duck into the public rest rooms there to wash your hands, arms, and face as soon as you finish filling your buckets, so you don't look like a blood-drenched serial killer, for starters (blackberry juice is startlingly bright), but also just in case you've rubbed up against some poison oak, which also grows down there in the thickets.

Searching for blackberries satisfies that primal hunting urge without the need for bait, rifles, or ugly camo vests. If you're the sort of person who gets antsy just sitting on the beach, this is your chance to bask in the sea, the sky, the sun, and the smell of pine needles on ocean breezes while still feeling productive. And if you want to feel even more fanatically productive, you can take your catch home and make jam.

Freshly picked blackberries are full of natural pectin, so they'll thicken into jam with minimal interference. If you're just going to make a small batch, you don't even have to buy special canning jars. Clean, empty jam or mayonnaise jars work fine, although you'll have to store the finished jam in the fridge and eat it within two weeks. The special trick here is to let the berries and sugar sit for a couple of hours before boiling. The sugar pulls the juice out, which then dissolves the sugar and turns the whole thing into something fruitful and mellow, bursting with ripe-fruit taste. Time, a little sugar, and a little dedication is all it takes; nature, even here in the middle of the city, gives you the rest.

Wild Blackberry Jam

4 cups blackberries

1 cup sugar

Rinse berries quickly and drain. Blot well. In a large, nonreactive bowl (try glass, enamel, or ceramic) mix berries with sugar. Cover and let sit at room temperature for one to two hours. Pour berries and liquid into a heavy, wide, nonreactive pot (stainless steel, enameled cast iron, or copper). Over medium heat, bring to a boil, stirring. When it starts to foam up, turn heat to low and keep stirring. At first it will produce a lot of frothy, magenta pink foam.

After about five minutes of simmering, the foam will subside and the mixture will start to thicken, turning glossy and deep purple. Scoop up a little onto a metal spoon. Tip the spoon and let the juice run off the side. When it begins to look sticky and slightly gummy rather than watery, it's done. The whole boiling process should take around 10 minutes. Remember that it will thicken more as it cools. Spoon into clean jars and fasten on lids. Let cool. Jars should be stored in the refrigerator and eaten within two weeks.