الأسبوع 27 · المحور الرابع، الوحدة الرابعة، المفهوم 4.2 «تغير مظاهر سطح الأرض» (ج): الرياح والكثبان ووادي الحيتان · 3 دروس
2
Wadi Al-Hitan: fossils in the layers
Discovery Education Science P4: Term 2, Unit 4, pp. 149–162
GoalRead rock layers from oldest to newest, explain what fossils tell us, and say how Wadi Al-Hitan shows that a sea once covered the land.
New words
fossil · the remains or marks of a living thing from long ago, kept in rockformation · a group of rock layers of the same kindancient · from a very, very long time ago
Explanation
Wadi Al-Hitan, the Whale Valley in Fayoum, is a World Heritage Site. Today it is desert, but the rocks tell a different story. Tens of millions of years ago, a sea covered this land. Sand, mud and shells settled in layers on the sea floor. As more layers piled up, their weight squeezed the lower layers until they hardened into rock. Rock made from settled sediment is called sedimentary rock, and sandstone and limestone are two examples. In a stack of layers that has not been disturbed, the bottom layer is the oldest and the top layer is the newest. Wavy lines in the rock show that moving water shaped the sand. The rocks hold fossils of ancient sea animals, among them whales. Scientists use the kinds of rock and fossils in each layer to work out what the place was like at that time. Many big skeletons in the lowest, oldest layer suggest a deep sea, and the sandstone and limestone above it suggest that the sea later became shallower.
Layer
Age
What it shows
top
newest
smaller sea creatures: shallower sea
middle
in between
sandstone and limestone made in a shallow sea
bottom
oldest
big sea animals, such as whales: deeper sea
Worked examples
Layer
Position
A
top
B
middle
C
bottom
Layer puzzle. Mina is shown three layers named A at the top, B in the middle and C at the bottom. She says C is the oldest, because new layers are put down on top of old ones. A is the newest.
shell in a desert rock→shell animals lived in the sea→a sea once covered the land
Clue from a fossil. Seif finds a rock in a desert with the imprint of a seashell. He says: a sea must have been here once, because shell animals lived in the sea.
Layer
Fossils
Idea
upper
small creatures
shallower sea
lower
large animals
deeper sea
Same story in two layers. The lower layer holds many skeletons of big sea animals. The upper layer is sandstone and limestone with none. Scientists suggest that the sea was deep at first and became shallower later, because sandstone and limestone form in shallow seas.
Class activity
Layer cake. Build a model of rock layers in a clear cup: first put in a spoon of coarse sand (oldest), then flour, then rice (newest). Draw it and label the oldest and newest layers. Put a small toy shell or a pasta shell in the middle layer for a “fossil”. Draw the cup again as a scientist would, and write what the fossil tells you. Do not eat the materials.
Practice
1
In a stack of rock layers, which layer is the oldest?
The bottom layer.
2
What is a fossil?
The remains or marks of a living thing from long ago, kept in rock.
3
Fossils of sea animals are found in the desert of Fayoum. What does this tell us?
A sea once covered this land.
4
A cliff has 6 layers. Two are limestone and the rest are sandstone. How many sandstone layers are there?
