Donald Trump is set to become the first US President in half a century to scale back land protections for national monuments, slashing the size of two protected sites in Utah.

Key points: Grand-Staircase Escalante national monument to be cut in half

Grand-Staircase Escalante national monument to be cut in half Bears Ears national monument to be reduced by 85 per cent

Bears Ears national monument to be reduced by 85 per cent Described as the largest rollback of federal land protection laws in America's history

Mr Trump's declaration to reverse land protections has been described as the largest rollback of federal land protection laws in America's history.

Both sites — Bears Ears national monument and Grand-Staircase Escalante national monument — feature thousands of Native American artefacts, including ancient cliff dwellings and petroglyphs.

What are national monuments?

Grand Staircase-Escalante national monument is 8,000 square kilometres in size. ( Unsplash: Patrick Hendry )

They protect areas of land and or water — not individual structures, statues, or memorials.

National monuments are public land, and though similar to national parks, have a few key differences.

The main difference is how they were established. In the US, Congress is able create any kind of national park site by passing legislation designating the specified park.

However only a US president has the power to create a national monument, using the authority granted to them in the Antiquities Act of 1906.

Mr Trump's announced changes to both Bears Ears and Grand-Staircase Escalante national monument. ( Flickr: US Bureau of Land Management )

Thus a national monument is generally created by the executive branch, rather than the legislative branch of government.

But a president can only create a national monument from land already owned by the federal government — so no state-owned or privately owned land.

A number of federal agencies have the power to manage national monuments — sometimes monuments are jointly managed by a few agencies.

Those agencies include the US Bureau of Land Management, the US Forest Service, and the US Fish and Wildlife Service.

As of May 2017, 16 presidents have created 157 monuments.

What Trump's scale-back would look like

Cedar Mesa Grand Gulch in Bears Ears national monument. ( Flickr: US Bureau of Land Managament )

Mr Trump has announced changes to both Bear Ears national monument and Grand-Staircase Escalante national monument.

Grand-Staircase Escalante is 8,000 square kilometres of desert-like landscape, designated a national monument by former president Bill Clinton in 1996.

Under Mr Trump's plan that area would be cut in half.

Both areas are rich with resources — when Mr Clinton announced Grand-Staircase Escalante as a monument, a coal mine was prevented from being developed in the area

Grand Staircase-Escalante refers to a large sequence of sedimentary rock layers stretching south. ( Unsplash: Patrick Hendry )

Environmentalist Emily Bleyl said she feared Mr Trump's decision would re-open that door.

"I think that they [the monuments] are national treasures in terms of the antiquities, in terms of the open space, the multiple uses that can be applied to the area," she said.

"I [would] hate to see it exploited, developed or fracked for the minerals."

Indian Creek in Bears Ears is a prominent climbing area. ( Flickr: US Bureau of Land Management )

Bears Ears national monument measures over 5,000 square kilometres and is full of cultural resources that reflect its historical and cultural significance to a variety of native American peoples.

It would be cut by 85 per cent under Mr Trump's plan — reversing protections made under former president Barack Obama in 2016.

Cedar Mesa Citadel ruin, an ancient Anasazi fortress carved into the sandstone cliffs of Bears Ears. ( Flickr: US Bureau of Land Management )

Named for a pair of buttes that rise over 600 metres above the surrounding valley, it is home to several distinct Native American tribes.

It is filled with culturally significant sites such as Newspaper Rock, carved with ancient petroglyphs recording over 2,000 years of history.

Newspaper Rock at Bears Ears is covered in hundreds of rock drawings. ( Reuters: Andrew Cullen )

Shaniah Chee, who belongs to one of the local tribes, said they were ready to fight.

"Bears Ears is a main resource for the native tribes down there, including Navajo, that's where I come from," Ms Chee said.

"It provides us with wood for warmth and medicine and overall is a sacred site."

Bears Ears is named for a pair of buttes that rise over 2,000 feet above the surrounding valley. ( Flickr: US Bureau of Land Management )

Thousands of sites 'under threat'

Environmentalists and some native nations said Mr Trump's decision would threaten around 100,000 sites of archaeological importance.

During his announcement Mr Trump declared that both areas were open for "public use".

"Here and in other affected states, we have seen harmful and unnecessary restrictions on hunting, ranching, and responsible economic development," he said.

"We have seen grazing restrictions prevent ranching families from passing their businesses and beloved heritage on to the children, the children that they love.

"We've seen many rural families stopped from enjoying their outdoor activities, and the fact that they've done it all their lives, made no difference to the bureaucrats in Washington."

Mr Trump's decision could threaten around 100,000 sites of archaeological importance. ( Flickr: US Bureau of Land Management )

He received immediate praise from state leaders who said the protections were too broad, and closed off the area to energy development

But Mr Trump's decision will not be as easy as signing a piece of paper — environmental and Native American groups are filing lawsuits in a bid to halt the move.

Darren Purcell, an associate professor in geography at the University of Oklahoma, said many of the Native American nations who were behind the expansion under Mr Obama would be suing the Government.

"The Navajo is going to sue and various conservation groups are also going to take this to court," he said.

"It's not a fait accompli, this is not necessarily a slam dunk."

Mr Trump declared that both areas were open for "public use". ( Flickr: US Bureau of Land Management )

According to the National Park Service, no president has ever tried to eliminate a monument, but some have reduced or redrawn the boundaries on 18 occasions.

The most recent instance came in 1963, when former president John F Kennedy slightly downsized Bandelier national monument in New Mexico.