注释：① channel vt. 引导，开导；形成河道 n. 通道；频道；海峡
[ 过去式channeled或-nelled 过去分词channeled或-nelled 现在分词channeling或-nelling ]
词组：
marketing channel 销售渠道，行销通道
main channel 主通道
change the channel [美国俚语]转移话题
english channel （英法之间）英吉利海峡
distribution channel 分销渠道；销售渠道
communication channel 通信信道；通信电路
river channel 河道；河槽；船舶航道
例句：
1. Mr Obama's speech was broadcast live by only one cable TV channel with limited reach.
奥巴马的演讲只通过一家有线电视频道在有限的范围内现场直播。
2. But after his landslide 10 years earlier he could have crossed the English Channel on foot.
但在10年前以压倒多数胜选上台后，他本可以“信步”跨越英吉利海峡。

② presence n. 存在；出席；参加；风度；仪态
词组：
in the presence of 在…面前；有某人在场
in presence of 提示
physical presence 实体存在；实际存在
military presence 军事存在
internet presence 网上广告
commercial presence 商业存在
例句：
1. I mean by that , your presence will become meaningless .
我的意思是，你的存在会变得没有意义。
2. For these the presence of people is central to the experiment.
由此说来，人类的存在正是这项实验的核心。

③ impressive adj. 感人的；令人钦佩的；给人以深刻印象的
[ 比较级more impressive 最高级most impressive ]
例句：
1. At first glance, all this has produced some impressive results.
初看之下，这些措施都产生了明显的效果。
2. This is the most impressive architecture I've seen on this trip.
这是我此次旅行见到的最令人难忘的建筑。

句子讲解：

1. And every time they come across the landscape, they are capable of eroding that landscape at rates that are perhaps fractions of an inch per year, meaning that over the course of one glacial cycle, you perhaps erode ten, twenty feet of rock.

　　【结构简析】整个句子是冰川每年侵蚀大约一英寸土地来做解释说明的。主句为"they are capable of eroding that landscape at rates"。

【参考译文】只要这些融化了的雪水到过的地方，每年就会侵蚀大约一英寸地，这就意味着，在一个漫长的冰期旋回过程中，可以侵蚀掉岩石十到二十英尺深。

　　2. They hitchhiked at the bottom of the glacier down the frozen valley, when the last Ice Age came to an end and the glaciers melted about 10,000 years ago, the boulders were left behind.

【结构简析】 我觉得这个"hitchhike"这个词用的非常好，这些石头就像搭便车的人一样，这个比喻迅速就让人明白是怎么一回事啦。很形象。

　　【参考译文】当最后一个冰河世纪结束的时候，巨石搭乘冰川的顺风车来到了结冰的峡谷，冰川早在一万年前融化，巨石则留在了这里。