Syncline
/ˈsɪŋklaɪn/
Definition 1 of 1
Pro Tip 1/3Geological Term
Used when discussing geological formations, particularly the structure of rock layers. 
I need to analyze the syncline to assess the age of these rock formations.
Pro Tip 2/3Visual Shape
Visualize a 'U' or 'V' to remember the shape of a syncline in the rock layers. 
The valley is bordered by hills creating a syncline shape.
Pro Tip 3/3Ancient Rocks
Remember that synclines are characterized by the oldest rocks being at the sides. 
Those synclines have the oldest sediment layers on their flanks