AC Transit’s new 31 bus line stitches together the old 39 and 21 lines to create a unique sea-to-sky experience between 14 feet elevation at Bay Farm Island, on the Bay shore, and 1516 feet elevation at Chabot Observatory, in the Oakland Hills. Its uniform 30-minute service interval makes side trips predictable at any stop for geologizing (or any other leisure pursuit).

It’s most convenient to board the 31 at Fruitvale BART station, near the route’s midpoint. Both directions are journeys into big-sky space with geology along the way.

Here’s the route south to Bay Farm Island superimposed on the street map:

^{Route map via gmap-pedometer.com}

and superimposed on the geologic map. (Click either to open it in a new tab.)

^{From USGS MF-2342; west tip of Bay Farm Island added by me. Qds/Qms, Merritt Sand; Qhaf/Qhaf1, young alluvial deposits; Qhb/Qhbm, bay mud; af, artificial fill.}

The route south toward the Bay starts by angling to 29th Street and then crosses first the railroad tracks, then the I-880 freeway, all in the delta of Sausal Creek. The views progressively widen until the Park Street Bridge across the Tidal Canal to Alameda. The other side of the bridge is on the ice-age dunefield beneath old Alameda, the sister to early Oakland’s Merritt Sand. The route runs the length of Park Street past its many tempting shops and nearby stops.

^{Chochenyo Park}

The bus turns left onto Otis Street with a detectable drop in elevation, then runs along the original shoreline passing the lagoons that mark its location. Everything on the Bay side is on artificial land created in the 1950s.

People like me, who are used to orienting themselves with natural landmarks, may find Alameda disorienting. For them the Bay Farm Island bridge, an important connection since Alameda’s earliest days in 1854, offers reassuring views of the Oakland Hills and the San Francisco skyline. Entering Bay Farm Island, the bus drifts through a dreamland of posh 1980s suburbia with backyard lagoons, built on former salt marsh. It pauses by the ferry landing at Shoreline Park, perched on a high rock sea wall in the steady Bay wind. All the while, the bus has been approaching the Oakland airport until the rising jets pass right offshore, their whining jets punctuating the stillness of this remote, breezy stretch of coast.

The bus loops back here after tracing the invisible north edge of the island’s ice-age dunefield, where vegetable growers gave Bay Farm Island its name in Gold Rush times. At Bay Farm’s small legacy town center, Maitland Drive and the Maitland Market recall Lester Maitland, the right-stuff Army pilot who made the world’s longest flight over open ocean, from Oakland Airport to Hawaii, a hundred years ago.

^{Plaque at 9351 Earhart Road, Oakland}

During the return leg on Park Avenue, the high Oakland Hills can be seen to emerge and rise as the bus crosses the extremely gentle mound of the dunefield; Central Avenue marks the high point at 33 feet elevation. Crossing the Tidal Canal on the Park Street Bridge returns us to Oakland’s alluvial land.

Possible short side trips from bus stops:
Park Street Bridge (23d/29th Aves)
Chochenyo Park (Park St/San Jose Ave)
The lagoons (Otis Dr/Park St)
Bay Farm Island pedestrian bridge (Otis Dr/High St)
Lagoon paths from Aughinbaugh Way (Aughinbaugh/Lagoon)
Leydecker Park (Mecartney Rd/Belmont Pl)

Here’s the route north to Chabot Observatory superimposed on the street map:

and on the geologic map.

^{Qpaf/Qpoaf, ancient gravel of the Fan; Kfn/JKf, Franciscan rocks of the Piedmont block; Jsv/Jb/sp, rocks of the Coast Range ophiolite; KJk/Kjm, rocks of the Great Valley Sequence}

The route toward the hills leads up the nearly imperceptible grade of Sausal Creek’s narrow floodplain, spine of the historic Fruitvale and Dimond neighborhoods. Low but distinct hills gradually rise on either side between Foothill Boulevard and the heart of the Dimond district, where the bus turns right on MacArthur Boulevard. These hills, known on this blog as the Fan, are ancient uplifted gravel that flanks the uplifted bedrock body under the city of Piedmont and the Oakmore neighborhood.

With that turn, the bus route changes abruptly from a gentle glide to a steady grinding ascent up Lincoln Avenue, Joaquin Miller Road and finally Skyline Boulevard. Lincoln is a ridge road that climbs the steep upper part of the Fan, first touching bedrock just above Alida Street. On the left, the Head-Royce School occupies a former rock quarry in the headwaters of Whittle Avenue Creek.

Just past the LDS temple at the top of Lincoln, the road crosses the Hayward fault.

Above the fault, the rocks change profoundly from the simple sandstone below it. The area in and around Joaquin Miller Park exposes it in all its variety, too much to summarize here, though not much is visible from the bus until roadcuts near the top reveal the dusty blue-green of our local serpentine rock. As the bus turns left up Skyline, look sharp for the first decent views over the Bay, and Serpentine Prairie and the hills beyond.

Soon a set of hairpin turns brings the serpentinite right up to the window. After that the route winds through the redwood-nourishing sandstone of the Joaquin Miller Formation past the entrance to the grove at Roberts Recreation Area, the tall trees hiding all but the sky overhead — an appropriate lead-up to the Chabot Space and Science Center and its weekend planetarium shows and telescope nights, finally well served by public transit.

The return leg is especially good for views of the Bay and high hills. This is the only bus route in Oakland with enough change in elevation to feel in your ears.

Possible short side trips from bus stops:
Peralta Hacienda, Jungle Hill and the Carrington Steps (Fruitvale Ave/Logan St)
The McKillop landslide scar (Fruitvale Ave/School St)
Dimond Canyon Park (Fruitvale Ave/MacArthur Blvd)
Hayward fault trace and London Road landslide (Lincoln Ave/Maiden Ln)
Burdeck serpentine belt (Joaquin Miller Rd/Hedge Ln)
Butters Canyon (Joaquin Miller Rd/Butters Dr)
Joaquin Miller Park (Joaquin Miller Rd/Robinson Dr)
Serpentine Prairie (Joaquin Miller Rd/Skyline Blvd)
Roberts Recreation Area (Skyline Blvd/Roberts Park)
High hills crest trails (Chabot Observatory)

This entry was posted on 15 September 2025 at 7:59 am and is filed under Oakland geology walks. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.