Warm stretch-fleece base layer made for cold days and mixed output.The Kyanites – our warmest base layer tops – are the choice for cold days and stop-and-start activities. On the move, the Polartec Power Stretch Pro fleece stretches for unrestricted freedom. During rest phases, or when it’s really cold out, the soft brushed interior traps heat to keep you warm. The fleece’s durable nylon outer has a smooth face for comfort under layers, and thumbholes hold the sleeves in place. The Kyanite Base Layers were previously known as the Rho Heavyweight base layers.