boulder 125 1/4 zip base layer top from artilect. the boulder 125 1/4 zip is a versatile, thermoregulating base layer from artilect. it's designed to offer effortless performance in a modern fit. using an ultra-lightweight nuyarn® 125 g/? merino fabric with matching strategically placed eyelets under the arms for increased breathability, there is no better combination of performance, comfort and durability in the world.