The snow was piling up in such packed layers, it had to be chipped loose with blows from picks.

白雪堆成十分坚硬的冰层，简直要用尖利铁锨来弄开它。

Blood-brain barrier is a tightly packed layer of endothelial cells that restricts free access of molecules between the blood and the brain.

血脑屏障是一层紧密堆积的内皮细胞，它限制了自由出入血液和大脑之间的分子。