Rock gets brittle when it gets cold, you don't need near as much force in your swing. If your rock goes below freezing and it has moisture on or in it, the freezing will cause micro fracturing which becomes macro fracturing when you hit it. That is why the ancients mined or picked up from non freezing creek bottoms and they usually spalled and bifaced on the spot, they didn't want to carry something back that would not be suitable for tools.
