1. The bigger animals went to the surface to drink, and were promptly bogged in the river.
    这个大动物到冰表面去饮水,突然掉进水里.

2. Apparently what happened was that water collected on these tar pits, and the bigger animals like the elephants ventured out on to the apparently firm surface to drink, and were promptly bogged in the tar.
    显然，事情的经过是这样：沥青坑里积存了水，大象那样的大动物冒险到似乎坚固的水面上去饮水，立即掉进了沥青坑。