A Neolithic monument has been discovered less than two miles from Windsor Castle. Dating from 5,500 years ago, it is one of the earliest known examples of monument-building in Britain.

A ceremonial gathering place known as a causewayed enclosure has been revealed with the discovery of a series of encircling ditches, artificial boundaries with gap entrances, at a vast site in Berkshire.

Archaeologists have found extensive quantities of animal bones as well as decorated pottery sherds, and evidence that pots were deliberately smashed, perhaps as festivities came to a boisterous close. Other finds include finely worked, leaf-shaped flint arrowheads, serrated blades, stone axes and grinding stones.

About 80 Neolithic monuments have been identified in Britain, but archaeologists are particularly excited by this one as they expect to uncover the entire circuit of the enclosure. Specialists from Wessex Archaeology made the discovery at a sand and gravel quarry near Datchet, within sight of Windsor Castle.

John Powell, a fieldwork director for Wessex Archaeology, said: “This is an exciting find. We’re talking about 5,500 years ago. These are earliest peoples, who are actually settling down in the landscape and leaving their mark on it. So it’s quite an impressive thing to be able to excavate.

“Often when they’re found now in modern conditions, you might only find part of the circuit. On this site, its position within the quarry means that we’ll be able to investigate pretty much all of the enclosure. So that will mean we’ve got a much better picture and an understanding of the site as a whole.”

The monument is on a slightly raised area in what may have been a marshy or seasonally wet landscape within the Thames floodplain. Powell said the area had been settled for thousands of years. “So we were going there expecting to find prehistoric remains, but we didn’t expect to find such a large monument on the site. We’ve worked on a few other quarries in the area and found small settlements, evidence of field ditches and small houses, but not these larger communal monuments.

“Some causewayed enclosures don’t contain much in the way of artefacts, whereas this one seems very rich in artefacts, which will be significant for the understanding of the early Neolithic in Britain.”

The pottery sherds have delicate decorative marks that may have been created with bird bones or twisted cord. The archaeologists plan to test deposits found in the vessels, to learn what was cooked in or drunk from them.

Pottery fragments from the site. Photograph: Handout

Powell said other complete monuments had been found before, including the nearby Staines enclosure, but that was only partially excavated in the 1960s. “It’s now all been destroyed because the M25 runs over the top of it. Now this is a chance to excavate an enclosure under modern conditions and using modern techniques.”

Excavations conducted so far suggest the monument was oval in shape with a projected perimeter of 500 metres (1,640ft), larger than an athletics track. Currently 265 metres of the enclosure’s arc – 12 ditch segments – have been traced, and the remainder could be uncovered later this year.

One theory is that it was seasonally occupied, with communities gathering for ceremonial feasting, exchanging goods and social obligations.

The excavation of this monument will add so much to our shared human story Roland Smith, Berkshire Archaeology

Human remains have also been found at the site. Jacqueline McKinley, Wessex Archaeology’s principal osteoarchaeologist, said the bones showed evidence of postmortem human manipulation of a body. “The skull and left femur had been removed from one individual and cut marks are visible on a skull that had been deliberately placed on the bottom of the ditch. Further work may allow us to understand both the treatment and shifting roles played by the dead in Neolithic communities.”

Asked about the animal bones, Powell said: “Cattle dominate the assemblage and we also have pig and sheep or goat from the domesticated farming side. Wild species are also present and include red and roe deer, as well as some fox bones.”

He said these suggested the hunter gatherer was settling down, bringing farming into the area, domesticating animals, making pottery and mining flint.

The archaeology is part of the planning conditions on the quarry, which is owned and run by Cemex UK, a supplier of cement, ready-mixed concrete and asphalt. The archaeological programme is monitored on behalf of the local planning authority by Berkshire Archaeology.

Roland Smith of Berkshire Archaeology said: “This is such an exciting and important discovery in the royal borough. The excavation of this monument will add so much to our shared human story, especially [about] this pivotal time in the earliest years of farming in Britain.”

Archaeologists have found remains from several periods of prehistoric, Roman and later activity. People are thought to have periodically lived or gathered in the area since the end of the last ice age, a period of 12,000 years.