To: Mt Olympia +2000 feet, 3 mile one-way
To: North Peak +2800 feet / -200 feet, 4 mile one-way
To: Main Peak +4200 feet / -500 feet, 6 mile one-way
Start from the Marsh Creek Road trailhead (elevation 900 feet, there is no sign that says Mount Diablo State Park but none that says "private property, no trespassing" either). Pass under the gated entrance, and follow a dirt road around a small hill on the right. Follow the road across a grassy saddle area, then into a pine woodland. You will see trail signs for "Mt Olympia trail". You can stay on the road or take the trail on the right side (more poison oak on the trail).
The trail will intersect the road again shortly. If you follow the road, take the right branch when you see the left branch going towards a private residence. The road then curves right through dense pines, reaching an open grassy area at the foot of the steep mountain on the left. Here the road starts to descend. Mt Olympia Trail starts right here on the left side.
A relatively short and very steep trail (nearly 30 degrees sustained gradient) to North Peak, an isolated high peak just 2 miles from the tourist-beaten main peak of Mount Diablo. Distance to North Peak is 4 miles, with 2800 feet elevation gain. This is my favorite local hike. On average you will see just one other person on the whole trip; between Mt Olympia and North Peak you'll see likely not see anyone. The trail passes through a variety of vegetation types and features far-ranging views in several directions (except southwestern side which is blocked by the main peak).
Mt Olympia is a small peak at 2946 feet elevation to the northeast of North Peak. The trail first climbs this peak steeply in about 3 miles. Do not turn off on any trails which says "Mt Olympia Road", that is a different trail primarily used by equestrians. Mt Olympia Trail is a hiker-only trail, being too steep and too narrow. It starts out in a dense brush area, immediately climbs steeply up the hillside. In a while the trail traverses open grassy slopes of about 50 degrees angle with hundreds of feet drop-off on the side. If it were snow covered it would be expert ski terrain. You will pass by large rock outcrops with spectacular views (one looks like an ideal campsite) perched on the slopes. The chaparral vegetation is fairly sparse here. In spring the rocks are often covered with mosses.
You climb up a gentler ridge with marvelous open views, then immediately resume a steep climb. Soon you reach Mount Olympia's summit at 2946 feet. You might find an (unofficial) summit register there in a tupperware box. Continue on the trail, which descends to the west of the ridge, bringing wide-ranging views in the northwest direction. It crosses a steep-sided grassy saddle, then traverses on the right side of a wooded buttress below North Peak (North Peak's summit transmitters are visible here). After some steep traverses in a dense Juniper and Bay forest, you top out onto the fire road leading to North Peak. Turn left to North Peak or right for the main peak (you must descend from here to reach the main peak).
The fire road leading to North Peak is the steepest you'll find anywhere. It's about 35 degrees, and the hard-packed road surface makes for very poor footing without cleats! Walking down this 200 feet long slope is especially treacherous. Scrambling the other side of North Peak is really a better alternative.
The fire road to main peak descends about 500 feet to Prospector's Gap, then ascends 800 feet to the main peak in about 2 miles.
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