Cyperus eragrostis
It sort of looks like a grass, but not quite. The leaves are thicker than your typical grass, and the stem is triangular in cross-section. The leaves are in threes, not in pairs as in grasses. And the big, robust seeds are nestled among long splayed leaves at the end of the stalk.
This is tall flatsedge, or Cyperus eragrostis–one of the most common sedges in Marin. Though partial to wet meadows, seeps, springs and stream banks, it can be found in in dry areas as well. Sedges (and grasses, and rushes) are particularly hard to ID since they have tiny flowers and require a lot of specialized terminology. For more information on how to key these plants, check here and here.
Flowers Of Marin 