The correct option is 1, 2 and 3.
Explanation
River terraces are surfaces marking old valley floor or floodplain levels. They are step-like landforms that flank the sides of river valleys, resulting primarily from the vertical erosion (downcutting) of a stream.
Statement 1 is Correct:
River terraces can be bedrock surfaces without any significant alluvial cover. These are known as bedrock terraces or strath terraces. They are formed when a river erodes laterally into the bedrock and subsequently incises vertically, leaving a flat bedrock bench at a higher elevation.
Statement 2 is Correct:
Terraces may also be alluvial terraces consisting of stream deposits. These are formed when a river cuts down into its own previously deposited floodplain sediments (alluvium). The remaining flat surfaces of the old floodplain, composed of gravel, sand, and silt, become the terraces.
Statement 3 is Correct:
River terraces physically indicate former river bed levels. They represent the elevation at which the river once flowed and deposited sediment before rejuvenation, tectonic uplift, or changes in sea level caused the river to cut downward to its current, lower level.
Key Takeaway:
River terraces are remnants of former floodplains formed by stream incision. They serve as geological markers of past river levels and can be composed of either bedrock or alluvial deposits.