The incident happened in the final five kilometres of Monday's stage from San Francisco to Aptos in Santa Cruz County and resulted in the duo, along with several other riders, being brought down.
Cummings required three stitches to close a deep cut in his right knee while the Englishman also re-injured his left wrist, which he broke in a crash six weeks ago.
Morabito sustained severe road rash on his left side which has left him unable to extend his body forward enough to grip the handlebars of his bike.
The pair were due to undergo X-rays to determine if any bones had been broken.
As well as the broken wrist he picked up at the Vuelta al Pais Vasco in April, Cummings broke his pelvis at the Volta ao Algarve in February and has already missed nearly two months of the season as a result.
"I've got experience at this now," Cummings said. "So if my wrist is broken again, I'll just go on the trainer and it won't take me long to come back."
Morabito, who finished ninth overall in California last year, was disappointed to have to throw in the towel.
"It happened so fast I didn't even have time to brake," Morabito said. "It was a huge road and there were only 60 guys so it was a case of being in the right place at the wrong time.
"California is always a good race for me," he added. "I really focused a lot on this race and was working hard to be in shape and ready for the last four days."