Ontario Provincial Police have reopened eastbound lanes of Highway 401 between Woodstock and Kitchener, after spending hours clearing a 20-car pileup that began near the Drumbo Road exit ramp.

OPP said there were no serious injuries.

The cause of the crashes has not been determined.

"Outside of people not driving according to road and weather conditions, that would be the early indicators of what happened," said OPP Sgt. Dave Rektor.

Wednesday's late-winter storm dropped an estimated 10 to 25 centimetres of snow in areas across southern Ontario, creating tricky travelling conditions.

Police initially reported that up to 60 cars were involved in the pileup. That number has since been revised to 20.