Due to the Sonoma Mountain's ancient volcanism, Penngrove is rich with obsidian and petrified wood.

According to the United States Census Bureau, the CDP covers an area of 4 square miles (10.4 km2), all of it land.

There were 1,120 housing units at an average density of 278.3 per square mile (107.5/km2), of which 61.5% were owner-occupied and 38.5% were occupied by renters.

The Mexican government granted Rancho Cotate to Captain Juan Castaneda in July 1844 for his military services in the region.

On Saturday at the Penngrove Fire Station, located at the corner of Main Street and Old Redwood Highway, the Rancho Adobe Firefighters' Association sponsors a breakfast.

Ruth Anderson, the famous "Bell-Lady" of Penngrove (who lived on the site of the old schoolhouse on Oak Street), recalls, "In the late 1860s, two brothers by the name of Penn came out from Pennsylvania and bought 80 acres (320,000 m2) of land in this area.

Others claim that the Woodward family, who came from Pennsylvania, named their property Penn's Grove in honor of their home state.

When the Northwestern Pacific Railroad was completed in 1870, the paving stone industry kept Penngrove station busy.

After the turn of the century, Penngrove became the "second largest egg and poultry producing area in the country.

To this day, many dilapidated chicken houses dot old farms and country roads in the area.

One of the oldest buildings in Penngrove, the old Methodist Episcopal Church was built in 1898 at the corner of Formschlag Lane and Petaluma Hill Road.

The original structure no longer exists, and a private home now stands on that property, however, the steps leading to "Eagle School" from Main Street are still visible.

In 1963, the present multi-purpose room was constructed and newer wings were developed between it and the older portion of the school.

In May 1895 the Petaluma paper reported: 'The Penngrove Post Office has had its name changed again and is now officially known by its former and best known title, Penn's Grove.'

The foundation of Firehouse #1 can be found between Penngrove Church and the Old Eagle School lot on Oak Street.

In 1975, the current firehouse was built at the corner of Old Redwood Highway and Main Street, the unofficial entrance to Penngrove.

Until the 1990s, a siren placed at the top of the building would sound in order to alert volunteer firefighters of a fire.

The siren had a greater than 1.5-mile (2.4 km) radius and could be heard frequently through the hot, dry summer months.

Currently, the Penngrove Firehouse is part of the Rancho Adobe Fire District, along with Cotati and other surrounding communities.

"During World War I, a group of Penngrove ladies met in Evart's Hall to do Red Cross work.