“In the beginning, all was dark.”

But in this context, “in the beginning” refers to “in 1959” and “all was dark” implies “there were relatively few Jews in Marin County.”

A mere 3,000 Jews were tallied in Marin County, according to a 1959 survey by the S.F.-based Jewish Community Federation, and JCF didn’t even bother dealing with Sonoma County.

But according to the recently finalized 2004 JCF study, nearly 50,000 Jews now call Marin or Sonoma counties home. In the last two decades, Marin’s Jewish population has grown by nearly 50 percent, while Sonoma County’s has rocketed up by 171 percent (at a time when the county’s general population grew by a third).

Yet, despite this bamboo-like growth, once the Golden Gate Bridge toll plaza is in your rearview mirror, the odds of finding a pair of married Jews grows rather slim. In both counties, the intermarriage rate is 75 percent.

Bruce Phillips, the demographer behind the JCF’s study, sees the astronomical rate less a symptom of suburban living than a case of self-selection.

“When you have a non-Jewish spouse, less of a premium is put on living in a Jewish area. It’s not that you’re looking to get away from [Jews], it’s just less important for both spouses,” said Phillips, a professor at Los Angeles’ Hebrew Union College and the University of Southern California.

Phillips likens the North Bay’s intermarriage rate to those in Orange County, Riverside or San Bernadino — all “new-growth areas on the fringes of the old metropolitan areas.”

And one of the major differences between the Jews of Marin and Sonoma counties is that moving into a Jewish area wasn’t important to Marin residents 40 years ago, while, for most Sonoma County residents, it wasn’t a concern in the last decade.

In other words, Marin is a far older suburb.

In Sonoma, 35 percent of Jewish households moved to the area in the last 15 years, according to the survey. In Marin, 41 percent did so between 1960 and 1979, “when that became the hip area,” noted Phillips.

With its relatively affordable properties, semi-rural locales and space to grow, Sonoma County is attractive for Jews for the same reason it’s attractive to non-Jews. And the Jews are coming from all over: Nearly a third of Jewish households are transplanted Southern Californians (an even higher percentage than in the South Peninsula). Eleven percent came from out of state and another 13 percent moved to Sonoma from Marin, where Phillips believes they were priced out.

To top it off, 56 percent of Sonoma County’s Jews have resided at their current address for three years or fewer.

“There’s a whole lot of moving going on,” quipped Phillips.

This highly mobile portion of the Bay Area’s Jewish community is also traditionally neglected. The federation closed down its Sonoma offices, leading to a short-lived North Bay federation in the 1990s, Phillips pointed out. A JCF office has since reopened, in Santa Rosa. (The Marin office is located in San Rafael.)

All of this points to challenges for the federation to reach out to these Jewish populations. But Phillips is optimistic. Despite other less-than-heartwarming numbers, Marin’s Jewish density is still something in the neighborhood of 6 percent, which is around triple the national average. And focus groups in Sonoma revealed that many Jews in the area were informally conducting their own Jewish activities such as Shabbat dinners or High Holy Day gatherings.

The prevalence of these informal Jewish gatherings led Phillips to insert new questions into the study, and “we found people are doing that all over the Bay Area.”

Other facts about the North Bay:

• In Marin County, 13 percent of Jews were born in the Bay Area, the lowest tally of any region surveyed.

• In Sonoma County, 63 percent of Jews polled believed they’d be in the “same city as now” three years down the road. This, too, is the lowest tally of any region. At 82 percent, Marin was the highest.

• In Sonoma County, 19 percent of Jews are singles younger than 40, the exact same percentage as in San Francisco.