Agglestone Rock is a sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 block of about 400 tonnes weight, perched on a conical hill, approximately 1 mile from the village of Studland

Studland

Studland is a small village on the Isle of Purbeck in the English county of Dorset. It is famous for its beaches and nature reserve. In 2001 Studland had a population of 480, the lowest in 50 years...

, south Dorset

Dorset

Dorset , is a county in South West England on the English Channel coast. The county town is Dorchester which is situated in the south. The Hampshire towns of Bournemouth and Christchurch joined the county with the reorganisation of local government in 1974...

 Legend has it that the devil threw the rock from the Isle of Wight

Isle of Wight

The Isle of Wight is a county and the largest island of England, located in the English Channel, on average about 2–4 miles off the south coast of the county of Hampshire, separated from the mainland by a strait called the Solent...

 with the intention of hitting Corfe Castle.

Geology

The rock is an "eroded relic of iron-cemented, Tertiary sandstone, the Agglestone Grit". It used to be anvil

Anvil

An anvil is a basic tool, a block with a hard surface on which another object is struck. The inertia of the anvil allows the energy of the striking tool to be transferred to the work piece. In most cases the anvil is used as a forging tool...

-shaped, but has since changed form due to erosion

Erosion

Erosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...

 and been toppled.

Visiting the Rock was listed by the Dorset Echo

Dorset Echo

The Dorset Echo is a daily newspaper published in the county of Dorset, England.The title publishes Monday to Saturday from editorial offices in Weymouth, and covers issues concerning south, central and west Dorset...

as one of the "25 Things To Do In Dorset Before You Die"

Mythology

" A musing stroll across the heath from Studland, brings you to the Aggllestone, the holy stone (Helig - Anglo-Saxon for holy) hurled by the devil on to the crest of a hillock rising above the peaty waste. Fiends often do dress like angels, and it is certainly hard to detect anything of the devil when the Madonna-blue chalices of that visionary flower, Gentiana pneumonanthe, are open on the heath. But devils did traffic with holy stones in archaic England, for devils were once gods themselves fallen from heaven upon evil days, the days when the usurping Celts looked with dread upon the works of their predecessors. For the Agglestone is a menhir".

Taken from
Downland Man by H.J. Massingham
Pub 1927 by Jonathan Cape

As listed on The Modern Antiquarian website