練習材料：
[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.
任務配置：L0+L1+L4
知識筆記：
shore line? ? 網絡 海灘線; 海岸線; 濱線; 灘線; 岸線;
the gentle slope? ? 緩坡
the continental shelf? ? 網絡 大陸架;
in the course of? ? [詞典] 在…過程中；在…期間;
sediment-covered? ? 網絡 沉淀物覆蓋的;
terraces? ? n. (相同的一排) 排房，排屋; (尤指房屋或餐館外的) 露天平臺，陽臺; (足球場的) 階梯看臺;? v. 使成階地（或梯田）;
tailing-off? ? 網絡 曳沙帶;
eroded? ? v. 侵蝕; 腐蝕; 風化; 逐漸毀壞; 削弱; 損害;
練習感悟：
