impassable; impervious;insurmountable; insuperable; not be overstepped;unbridgeable
❍ 到了十九世纪中叶，西方突破了中国孤立的墙壁，那在以前是～的。(《毛泽东选集》1401) Then in the middle of the nineteenth century the heretofore impervious wall of Chinese isolation was breached by the West.
不可逾越
impassable;indespensible;insurmountable
～的鸿沟impassable chasm/～的阶段indespensible stage/～的障碍insurmountable obstacle
不可逾越bù kě yú yuè
逾：越过。不能越过。impassable, insuperable, impervious, insurmountable