Marin County Parks and Open Space Department

This is a small preserve (50 acres), but it’s story is a perfect example of how many areas that are now open space were spared from development. As the town of Mill Valley grew in the late ‘60’s, the citizens realized that the surrounding ridges would soon be covered with buildings. Grassroots activism squelched the plans of several developers, and eventually MCOSD was able to acquire the first 50 acres, which form the eastern half of the preserve.

Alto Bowl DetailHorse Hill Preserve is actually the southern half of the Alto Bowl Preserve and is leased for horse pasturing by the Alto Bowl Horseowners Association. The sight of horses grazing peacefully on this grassy hillside has been a landmark for decades. Slated for development in the 1980s, the credit for its preservation goes to the horse owners, their supporters, the Marin County Open Space District, the Marin Community Foundation, and the City of Mill Valley who together raised 2.4 million dollars.

Dedicated volunteers have restored many acres of this preserve’s native habitats by waging an ongoing battle against French broom and other invasive, exotic plants. The Bob Middagh Trail connects the Alto Bowl Preserve with the Camino Alto Preserve to the west.


East access: from Hwy 101 in Mill Valley, take the exit for E Blithedale Ave east. Turn right onto Tower Dr, right onto Plaza Dr, veer left on Plaza, right onto Lomita Dr, and follow to end (dead end).

West access: Same as above, except MCOSD gate is on the right, before Lomita Dr ends.

Visit,,, or dial 511 to find out about alternative transportation and public transit options from your location.

Open Space Field Guide

A great place to look for butterflies patrolling the forest edges and gathering at the springs that dot the south slope. These springs also support a healthy population of California voles, which in turn support a variety of hawks.

eNature BirdsBirds
eNature ButterfliesButterflies
eNature ReptilesReptiles & Amphibians
eNature MammalsMammals
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Birds, Geologic Interest, Horse Trough, Wildflowers


24 hours

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