Story highlights The slab of weathered crude and sand was more than 160 feet long

Most of its 40,000-plus pounds was sand and water, a Coast Guard official says

Louisiana is the last state where Deepwater Horizon cleanup continues

Three years after the Deepwater Horizon oil spill, workers have dug up a massive chunk of weathered crude from the shallows off a Louisiana beach.

The tar mat -- a slab of oil residue mixed with wet sand -- was about 165 feet long by 65 feet wide, said Lt. Cmdr. Natalie Murphy, a Coast Guard spokeswoman. It weighed more than 40,000 pounds, but more than 85% of that weight was sand, shells and water, she said.

The mat was found under the surf off Isle Grand Terre, Louisiana, about 90 miles south of New Orleans. Crews using an excavator dug the largely submerged tar mat out of the beach in chunks over the last few weeks, Murphy said.

Louisiana is the last state where regular cleanup operations from the oil spill are still under way . The Coast Guard and BP, the owner of the ruptured well at the heart of the disaster, declared the cleanup over in Alabama, Florida and Mississippi earlier this month, but more than 80 miles of the Louisiana coast are still being patrolled.

BP has recovered more than 2.7 million pounds of waste from Louisiana shores in 2013, with residual oil making up between 5 and 15% of that total weight, company spokesman Jason Ryan told CNN.