翻译里的几个句子，老师帮我看下我翻译的合适不，哪里需要改进

　　1，这就是古代帝王通常在自己登基或晚年时到泰山封禅祭拜的原因

　　it's reason that emperors made pilgrimages mostly to Mount Tai when they were crowned or in their later years

　　2泰山拔地通天，气势磅礴(这句实在不知道怎么翻)

　　Mount tai has majestic scene ，like pull the groud into heaven.

　　3长江总长度为6300米，发源于青海省，向东流入东海。长江流域是中国最重要的农作物产区，粮棉产量分别占全国总量的40%和30%。

　　The river is about 6300km and originated from Qinghai province，eastward to the East China Sea.

　　The important agricultural base in China is the Yangtze river valley.

　　The productivities of grain and cotton account for nation's 40% and 30%