COLLINSVILLE, Ill. (AP) - An ongoing effort to get an ancient southwestern Illinois historical site designated as a national park is getting a boost from U.S. Senator Dick Durbin.

The Democrat and East St. Louis native on Thursday sent a letter asking President Obama to declare the Cahokia Mounds State Historic Site in Collinsville as a national monument. The request comes two years after the National Park Service began to study that prospect.

The property is designated by the United Nations as a World Heritage Site. It's also a national historic landmark, a lesser designation.

Cahokia is believed to have been inhabited from 700 to 1400 A.D. and was among the most complex societies of ancient North America.