A prominent landscape feature and a popular outdoor recreation area with cliffs that rise 300 feet (91 m) over the city below, East Rock is part of the narrow, linear Metacomet Ridge that extends from Long Island Sound near New Haven, north through the Connecticut River Valley of Massachusetts to the Vermont border.

Diabase is a dark colored rock, but the iron within it weathers to a rusty brown when exposed to the air, lending the ledges a distinct reddish appearance.

These diabase cliffs are the product of lava intrusions hundreds of feet deep that welled up through faults creating sills during the rifting apart of North America from Eurasia and Africa over a period of 20 million years.

Erosion and glacial abrasion over the subsequent 200 million years wore away the weaker sedimentary layers, under which the sill had intruded, at a faster rate than the diabase, leaving the abruptly tilted edges of the diabase sheets exposed, creating the distinct linear ridge and dramatic cliff faces visible today.

Cooler north facing backslopes tend to support extensive stands of eastern hemlock interspersed with the oak-hickory forest species more common in the surrounding lowlands.

[1] East Rock is a popular outdoor recreation destination among residents and visitors of the greater New Haven region.

Activities permitted in the park include hiking, snowshoeing, cross-country skiing, picnicking, bicycling (on roads and city-designated mountain bike trails only), boating (on the Mill River), bird watching, and dog walking.

The Trowbridge Environmental Center is open Thursdays and Fridays from 10:00 am to 5:00 pm, and at least one Saturday a month for public programs; it offers displays and information about the geology and ecosystem of East Rock.