From Sibley Volcanic Preserve’s north trailhead, Quarry Road climbs steep scotch broom-cloaked hillsides directly to the Round Top volcanic area. It’s a road all right, wide, paved at the outset, then gravel, not open to cars but a bit short of charm for the first half mile. That’s when you get to a short steep side trip (Pond Trail) down to a quite charming unnamed stock pond overgrown with cattails and populated by frogs, newts and blackbirds. There’s a picnic table there, should you want to linger.
Back on Quarry Road, keep climbing. I trust you brought the park brochure with its self-guided geology tour of the Round Top area, mostly along Volcanic Trail. You’ll be taking it backwards, which will do wonders for your cognitive agility. To foster the requisite state of calm alertness for this project, be sure to take in (or walk) the human-built labyrinths in the quarries at Markers 5 and 2.
To reach Marker 1 at the water tank, my suggested route is Loop Trail, which has nice views, but is a bit longer than Round Top Trail. Either one will bring you back to the Sibley Staging Area at Skyline Blvd, whence you may follow (single-track) Skyline Trail down Round Top Creek to our starting point.
Details: No fees. Bikes and leashed dogs are permitted (except no bikes on single-track part of Loop Trail or on Round Top Road to the top of Round Top); Sibley brochure/map/guided tour is on 2 pages: http://www.ebparks.org/Assets/_Nav_Categories/Parks/Maps/Sibley+map.pdf and http://www.ebparks.org/Assets/_Nav_Categories/Parks/Maps/Sibley+text.pdf . Accessible restrooms are at both parking areas. Hiking to the peak of Round Top is not difficult but thick eucalyptus and pine woods obstruct views and one sees mostly communication equipment there. Sibley Preserve is too steep and rocky to be very wheelchair-friendly, still I have traveled and enjoyed all the trails I mention except Loop Trail southeast of Round Top and Skyline Trail, which are too narrow for me. But Round Top Trail has a very serious case of excessive geology, crossing many yards of the kind of loose cobbles I ignorantly imagine Robert Frost used to construct his wall. I have horsed my long-suffering bones over it 5 or 6 times in my long-suffering wheelchair, for reasons that are difficult to remember, and if I never hike it again it will be too soon.