Smartwool 250 base layer quarter zip 100% merino wool Smartwool describes this top as: As one of our heaviest base layer tops and one of our best base layers for cold weather, this garment uses 100% responsibly sourced Merino wool, shoulder panels, and a flatlock seam construction to help prevent chafing as well as keep you comfortable and temperature regulated as you enjoy high-intensity skiing, hiking, running, and more. This top is machine washable on gentle/delicate cycle.