CHS 1 Hike - Little Si & Boulder Garden Loop

On this hike, our top priorities are safety, curiosity, and enjoyment. We'll strive to meet our goals for pace, distance, and elevation gain, while making sure we have a good experience together along the way—adjusting our pace to support one another, especially when the trail gets steep or challenging.

Trail: Little Si via Boulder Garden Loop
Geological Theme: Exhumed Roots and Glacial Sculpting

This trail offers a compact and vivid cross-section of the geologic forces that shaped the western Cascades. The hike begins in second-growth forest, weaving through moss-draped maples and Douglas firs, but it’s the rocks underfoot—and towering beside you—that tell the deeper story.

The Boulder Garden Loop, branching off early from the main Little Si trail, showcases massive erratics and jumbled talus—boulders left behind not by landslides, but by the glacier that once blanketed this valley during the last Ice Age (~15,000 years ago). Many of these boulders are rugged blocks of granodiorite and tonalite, intrusive igneous rocks that formed deep underground during the late Cretaceous to early Tertiary, when subduction off the Pacific coast drove magma into the crust beneath what is now the Cascades.

These plutonic rocks, now exposed, are part of the Mount Si Intrusive Complex—the exhumed roots of an ancient volcanic arc. Over millions of years, tectonic uplift and relentless erosion stripped away the overlying volcanic material, exposing these deeply crystallized cores. Some surfaces on the boulders still bear glacial polish or faint striations, evidence of their icy transport during the Vashon Glaciation.

As you rejoin the main Little Si trail and ascend the rocky shoulder of this smaller sibling of Mount Si, you’ll pass weathered cliff bands and outcrops shaped by frost wedging and exfoliation. Note how the trail cuts across tilted layers and joins, offering a glimpse into the bedrock’s cooling and fracturing history. The upper viewpoint reveals a dramatic look at the Middle Fork Valley, carved by a combination of fluvial incision and glacial overdeepening, framed by peaks that are both young and very, very old.

Look for:

• Talus fans formed by ongoing rockfall and freeze-thaw cycles

• Lichen-encrusted granite boulders that have been sitting since the Ice Age

• Sharp clefts and cracks that echo the region’s seismic and tectonic stress

This trail is a short walk through deep time—an ideal introduction to the hidden fire and ice that built the Cascades.

Meeting 7am at Carpool location. Carpooling is recommended, but let me know if you plan to meet us at the trailhead. Carpool volunteers are welcome.

Carpool Location: Tibbett’s Valley Park and Ride
1675 Newport Way NW, Issaquah, WA 98027
Off Newport Way, across from the Transit Center,
Next to the Skatepark.