The islands are under the jurisdiction of the United States with security provided by the town of Branford police and the US Coast Guard.

The archipelago of islands made up of Stony Creek pink granite bedrock were once the tops of hills prior to the last ice age.

The islands themselves - long prized by sailors on the Sound as a sheltered deep-water anchorage - comprise 23 that are inhabited (most of them wooded), numerous barren rocks and hundreds of reefs visible only at low tide.

Although they are said to be named for the thimbleberry, a relative of the black raspberry, that plant is seldom seen in the area, and is more frequent in northern New England.

In Reflections in Bullough's Pond, Diana Muir describes the important nineteenth century oyster farming industry that thrived around these islands.

As with most of southern New England, the ecology of the islands has been heavily influenced by thousands of years of intermittent human occupation.

One particularly intrusive event was the felling of all the trees on every island during the American Revolutionary War to eliminate hiding places for British ships.

The plant species of the islands were extensively studied by Yale botanist Lauren Brown; although the islands represent a unique ecological niche, combining a thin layer of soil, a high concentration of salt, and extreme exposure to weather, no unique, unusual, or rare plant species have been found; instead, the shrubs and trees represented are generally similar to those on the nearby mainland, selecting only those that reproduce by berries or other fruit carried by birds to the islands, for example raspberries, blackberries, sassafras, cherries, etc.

Poison ivy has established itself in many sites on the islands, in some places thick enough to forbid entire areas from human intrusion.

Other cultivated plants, such as ivy and some types of climbing roses, have established themselves on the islands to the point of becoming invasive species.

The exposed nature of the houses makes them dangerous during storms; local residents still talk about the hurricane of 1938, which killed seven people.

[10] Residents of the area tend to protect the privacy of island dwellers, obeying the 5-mile (8.0 km)-an-hour speed limit for motorboats and never landing without an invitation, though trespassers are often cited and ticketed.