In structural geomorphology, why do fold mountains commonly show an enormous thickness of sedimentary rocks in their structure?

Introduction / Context:
Fold mountains such as the Himalayas, the Alps, and the Andes are classic features of structural geomorphology and plate tectonics. One characteristic of these mountain ranges is that they consist largely of very thick sequences of sedimentary rocks that have been folded, faulted, and uplifted. This question asks why such enormous thicknesses of sedimentary rocks occur in fold mountains, testing your understanding of geosynclines and mountain building processes.

Given Data / Assumptions:

• The landforms in question are fold mountains.
• The observed fact is that these mountains include very thick layers of sedimentary rock.
• Options mention simple valleys, geosynclines, plains, and special types of folds.
• We assume the classical geosyncline theory as a useful explanatory model alongside modern plate tectonics.

Concept / Approach:
Before a fold mountain belt forms, there is usually a long lasting, deep, subsiding trough known as a geosyncline along a plate margin. Rivers and marine processes deposit huge quantities of sediments into this geosyncline over millions of years, building a very thick pile of sedimentary layers. Later, when plate convergence and compression occur, this thick sedimentary sequence is folded, faulted, and elevated to form a mountain chain. Hence, the great thickness of sedimentary rocks in fold mountains reflects the prior accumulation of sediments in a geosyncline, not simple valley deposition or folding of an old plain alone.

Step-by-Step Solution:
Step 1: Recognise that fold mountains form mainly at convergent plate boundaries where large horizontal compressive forces act. Step 2: Before strong compression, a long, narrow, deep basin or geosyncline exists, gradually subsiding and collecting sediments. Step 3: Over geological time, rivers, glaciers, and marine processes deposit vast amounts of sand, silt, clay, and limestone into this geosyncline, leading to a very thick sedimentary pile. Step 4: When plates converge more strongly, these accumulated sediments are compressed, folded, and faulted, eventually rising as a fold mountain belt. Step 5: This explains why fold mountains display thick sedimentary sequences; they are the deformed remnants of the geosynclinal fill. Step 6: Therefore, the correct explanation is that the sediments accumulated in a geosyncline before being folded into a mountain chain.

Verification / Alternative check:
Geological cross sections of the Himalayas or the Alps show deep piles of sedimentary rocks that were originally laid down in old oceanic or marginal basins. These basins are analogous to geosynclines in classical terminology. Modern plate tectonic models describe similar processes along continental margins and subduction zones, where thick sediments accumulate before being deformed. This consistent pattern in many fold belts supports the explanation based on prior sediment accumulation in geosynclines.

Why Other Options Are Wrong:

• Because sediments were deposited in a simple river valley for millions of years: River valleys do accumulate sediments, but they are too narrow and shallow to explain the enormous thickness seen in large fold belts.
• Because the old plains were folded directly into mountains without prior deposition: Plains represent a simpler landscape and do not by themselves contain the necessary immense sediment thickness.
• Because already solid rocks were folded into recumbent and nappe forms only: Recumbent and nappe folds describe shapes of folds, not the original cause of very thick sedimentary sequences.

Common Pitfalls:
Students sometimes focus on the visible stage of folding and forget that mountains have a long history before uplift. They may imagine that the crust simply crumples without considering the earlier subsiding basin that collected the sediments. To avoid this, always link fold mountains with earlier geosynclines or marginal basins where thick sediments accumulate before tectonic compression. This helps explain both the thickness and the predominantly sedimentary nature of rocks in these mountain ranges.

Final Answer:
Fold mountains have very thick sedimentary rocks mainly because a very thick pile of sediments accumulated in a geosyncline before being folded.