练习材料：
[Day 1777 2019-08-08]
Lesson 30-3 Exploring the sea-floor
From the shore line, out to a distance which may be anywhere from a few miles to
a few hundred miles, runs the gentle slope of the continental shelf, geologically part of
the continents. The real dividing line between continents and oceans occurs at the foot of
a steeper slope.
This continental slope usually starts at a place somewhere near the 100-fathom
mark and in the course of a few hundred miles reaches the true ocean floor at 2,500-
3,500 fathoms. The slope averages about 1 in 30. but contains steep, probably vertical,
cliffs, and gentle sediment-covered terraces, and near its lower reaches there is a long
tailing-off which is almost certainly the result of material transported out to deep water
after being eroded from the continental masses.
任务配置：L0+L1+L4
知识笔记：
shore line 网络 海滩线; 海岸線; 滨线; 滩线; 岸线;
the gentle slope 缓坡
the continental shelf 网络 大陆架;
in the course of [词典] 在…过程中；在…期间;
sediment-covered 网络 沉淀物覆盖的;
terraces n. (相同的一排) 排房，排屋; (尤指房屋或餐馆外的) 露天平台，阳台; (足球场的) 阶梯看台; v. 使成阶地（或梯田）;
tailing-off 网络 曳沙带;
eroded v. 侵蚀; 腐蚀; 风化; 逐渐毁坏; 削弱; 损害;
练习感悟：
