BAJA CALIFORNIA SUR, Mexico >> Chances are you’ll be luckier than I and you’ll get a warm day when you visit the ancient paintings at Baja California’s La Trinidad Cave.

But I was there in the cool of January and gritted my teeth as I dived into the cold river that meanders through a canyon of rock the colors of chocolate and cinnamon.

This is a rite of passage — quite literally — required to see the fascinating prehistoric images at La Trinidad, named for the three-peaked mountain above the canyon.

Guide Salvador Castro Drew’s email the day before was terse and slightly mysterious: “Meet at the town arch at 9 tomorrow and bring your swimsuit.”

That all became clear after a bumpy, four-wheel-drive journey across the desert and a 20-minute hike during which we examined petroglyphs at trailside.

The river wasn’t wide, but it was deep enough to require swimming to reach our destination. Giant boulders were conveniently placed to provide privacy as other tourgoers and I changed into our swim togs.

Splashing out at the far side, Patricia Berman, visiting from Corvallis, Ore., announced, “Whoo! I’ve been known to jump into mountain lakes, but usually I’ve been hiking all day and I’m hot and dusty and it’s 90 degrees out.”

But as we shook like wet dogs and climbed to the cave site, we saw that it was worth it.

RUSTIC FIGURES

The cave had collapsed some years earlier, but that didn’t obscure the rustic figures of animals, fish and humans in daubs of black, white and red that festoon the entry wall.

Salvador, a 53-year-old native of nearby Mulege, has devoted years to learning about the paintings here, offering guided trips to supplement his family’s ranch income.

The paintings vary in origin from 1,500 to 7,500 B.C., he said. Little is known about the original artists, but some were the indigenous Cochimi people, who fished in the sea and roamed inland for seeds and desert fruits.

White paint came from limestone. Black came from iron; red and yellow, from desert plants.

Some images are phenomenally well preserved, a credit to the dry climate and remote location. A red deer head looks as if it might have been stenciled on the wall last week. (Online experts credit the “Trinidad Deer” as the best prehistoric deer painting in Baja.)

Two fish shapes are next to what Salvador described, only half in jest, as “an ancient barbecue fork.”

A shaman figure perpetually holds his arms to the skies. White handprints dot the wall as if kindergartners had been playing with paint.

Children were given peyote and encouraged to add to the wall painting as part of ceremonies, Salvador said. Some, too young for peyote, would die. Others, who had hallucinations, might become shamans.

CAVE OF THE SERPENT

The next day I’m riding with Ivette Granados Marines, a 43-year-old guide with Sea Kayak Baja Mexico, a Loreto-based outfitter, in her friend Erika’s dusty old Pathfinder, which starts when you thread the key into the ignition that hangs by loose wires from the steering column.

We’re lurching along a rocky path when I am astounded to see a tarantula the size of my hand sedately crossing the track in front of us.

Nobody else thinks it’s a big deal.

We’re on our way to see more cave paintings, this time at “Cueva La Serpiente,” the Cave of the Serpent. It’s 35 minutes up the arroyo from San Javier, a village of 1,000 people in the craggy hills of the Sierra de la Giganta, 23 uphill miles from the seaside city of Loreto.

Ivette has been wanting to check out the cave as a possible new excursion for the guide service. I tag along.

We stop at San Javier, where Jesuits founded their second permanent mission in the Californias in 1699 (the first was Loreto, two years earlier). Here, natural springs turn the desert hills green.

Building materials for the primitive church came by mule from Loreto. Out back, the missionaries planted orchards. An incredibly gnarled 300-year-old olive tree still bears fruit, its trunk twisted like the rubber band that drives a child’s toy airplane.

Ivette and I follow a path through orchards to the home of her friend Erika Castanon Moreno, whose four-wheel drive we need to borrow. Her small home is of concrete blocks, with an open-air, palm frond-roofed dining area.

There is a chicken coop. Empty.

“They had chickens but a puma came,” Ivette explains.

Erika’s bright-eyed son, Esteban, 10, climbs a tree and plucks tangerines for us to take on our cave hike. We pile into the Pathfinder and stop to fill up at a nearby home where they sell gasoline from a bucket. Erika’s husband, Luis, sucks on a plastic hose to start a siphon.

At the wheel, Erika zigzags through the village, stopping every few feet to hang out the window and hail a friend.

She studied oceanography at a university, she confides to me.

“I am an oceanographer, and now I live here in the mountains. Lots of things grow here. The weather is a little different, and there is mucho agua — lots of water!”

Erika used to be a kayaking guide in Loreto.

It is 5 degrees Celsius (or 41 Fahrenheit), the coolest all season, Erika announces as we head out of town on a rough road. We soon splash across the dribbling stream of Arroyo Santo Domingo.

“In storms the water is this high,” she indicates, holding a hand to the car’s roof. Hurricane Odile socked this area in 2014, she adds. “Everybody slept in a big room at the school. We were all friends that night!”

Near the trailhead we stop at Rancho Santo Domingo to meet the straw-hatted rancher, Humberto Verdugo Garcia, and his wife, Raquel Morillo Talamantes. Humberto will be our guide and, because the cave is on his ranch, will collect the 100-peso fee set by the government.

We drive behind him a few minutes, yielding momentarily to that tarantula, then rock-hop up a talus hillside to the cave.

There are fewer paintings here. Some have been obliterated or partially hidden by rockfall. There is a snake image, very faded. (Another “Cave of the Serpent,” to the north of this, has more spectacular snakes with — inexplicably — deer antlers.)

But here there is a nice image of a spouting whale. The triangular whale spouts look like “early margarita glasses,” Humberto jokes.

Back at the ranch, Humberto and Raquel invite us for coffee under their palapa.

“It is ranch coffee! With goat milk!” Ivette whispers to me.

The milk is rich and light yellow. There’s a tub of coarse sugar. It’s deliciously reviving.

Humberto tells us that he and Raquel can also give tourists a lunch of roast goat, with goat cheese and other sides. Ivette suggests she might add it to her excursion.

We leave with big smiles and lots of handshakes.

HARDY INHABITANTS

The next day I am on a mule.

I booked a half-day ride near San Javier with mule wrangler Trudi Angell, who came here from California’s Napa Valley in the 1970s. Now she has dual citizenship.

For years she ran a kayak guide service called Paddling South. These days she runs a string of mules, with a new business name: Saddling South.

Trudi’s 27-year-old daughter, Olivia, and their Weimaraner dogs, Luna and Hershey, accompany us on a ride through the high desert of cactus, mesquite, aromatic wormwood and desert lavender.

Mules are the hardy work animal of choice in Baja. Trudi sets her mules loose to fend for themselves in the hot summers here because they are so adept at finding food and water.

“They’re in the natural-foods store when they’re out in the desert!” she quips.

Trudi likes to take visitors to the small ranches near San Javier to meet the locals, who are likewise hardy. (A 2013 photo book, “The Bare-Toed Vaquero,” from University of New Mexico Press, delightfully documents this self-sufficient breed of “rancheros.”) What Trudi calls the “Goat Cheese Ride” is one of her most popular outings, with a stop at Rancho Viejo.

We stop in to meet the lady of the ranch, Maria del Rosario de los Santos de Romero. She just goes by “Chari.” She shows us a batch of goat cheese in the works beneath a shelf weighted with big rocks to squeeze out the whey.

“This is where they will bring out the goats for milking and they give a cheese-making lesson,” Trudi tells me.

Olivia shows me the ranch’s old stone-walled corrals and a pen of bleating baby goats, which are among the more adorable creatures on the planet.

We leave the ranch and amble along old jeep trails on our mules, Raton, Chino and Dulce, as we gaze up at the high Mesa San Geronimo. From its top, Trudi tells me, you can see both coasts of Baja, which is 50 miles wide here. We see quail, a red-tailed hawk and turkey vultures.

I’ll end the day back in a cozy hotel at seaside in Loreto. Over a cold beer I’ll tell strangers how I’ve fallen in love with the raw beauty of Baja’s high desert. The giant spider they can keep.