Southern Ontario's first significant snowfall of the winter season claimed two lives Friday when a 28-year-old mother and her three-year-old son were killed in a six-vehicle crash in Caledonia.

Police say 28-year-old Amanda Fleming and her son Denver Sherk died Friday afternoon after their SUV crossed the centre line on the two-lane Highway 6 Caledonia bypass and was hit by a transport truck.

The SUV then struck four other vehicles. Fleming was ejected and pronounced dead at the scene. Her son died later in hospital.

The other five drivers weren't hurt.

Police say the SUV was traveling north approaching the Grand River bridge when it somehow moved into the southbound lane. The SUV was struck by the transport truck and then spun out.

"As a result of that impact, it struck four more vehicles that were on the bridge at the same time,” OPP Const. Mark Foster explained to CTV Toronto.

"We'll look at road conditions at the time, the vehicle itself and anything else that any of the other involved drivers will be able to tell us."

Winter weather was the cause of another crash at approximately 4:30 a.m. Friday, on the northbound lanes of Highway 400 just north of Highway 89. A tractor trailer jack-knifed. A second car flipped over, landing on its roof. A woman was rushed to hospital with critical injuries.

OPP Sgt. Dave Woodford says the weather condtions likely played a role in the accident.

"We had a light, light rain and all of the sudden, the temperatures dropped and it was -5 Celsius. We had a flash freeze up here,” he told CTV.

Highway 400 was closed for five hours while police investigated.