"This is an extremely shocking case for us," said Chief Timothy Anderson of the park's police department.
The motive remains unclear, but Anderson said that Newton had a history of depression and appeared to have self-inflicted knife wounds on her wrist.
Police were called at 10:30 a.m. Saturday on reports of a damaged Honda sedan in the Del Valle Regional Park east of San Francisco. They found the car abandoned with the engine running and an empty child's seat.
Officers were about to tow the car by 12:30 p.m., believing it was involved in a hit-and-run, when Newton walked up to them holding her lifeless baby, Anderson said.
Officers attempted to resuscitate the baby, but Anderson said they were unsuccessful.
Newton was placed under arrest after she made statements implicating herself in the stabbing, Anderson said, adding that investigators believe they found a knife in the area that was used in the alleged crime.
The stabbing occurred in a remote part of Camp Arroyo, the site of a popular children's camp inside the 4,400-acre park south of Livermore, a suburb that's about 45 miles east of San Francisco.
Newton remains in jail with no possibility of release on bond, police said. She is due in court on Monday.
Information from: San Jose (Calif.) Mercury News, http://www.mercurynews.com