It is the accumulation of many sheets of impact ejecta from many craters, large and small. There the impact layers are more jumbled. The answer is our old friend, Carbon Dioxide . When a large meteor hits the Moon, it blasts debris around, but there is no real difference between the debris from one impact or another. All end up in an undifferentiated vertical stack.
Water ice is relatively refractory.
Some of it melts or vaporises in the impact, but most is blasted out as unmelted grains. The CO2, on the other hand, is very volatile. It readily forms a gas with much lower energy requirements. Thus each impact would lay down an ejecta sheet, and then a layer of CO2-rich ices. The process of collapse of these temporary atmospheres would have been spectacular.