ASBOG Practice Exam 2025 – Complete Preparation Guide

The law of initial horizontality is a fundamental principle in geology that states sedimentary layers are originally deposited in a horizontal position. This principle implies that when sediments settle under the influence of gravity, they tend to accumulate in flat, horizontal layers. It serves as a key reference point for geologists when interpreting the geological history of a region, as any tilting or folding of these layers that is observed later can indicate subsequent geological events such as tectonic activity.

While sedimentary layers can indeed be displaced, not all are, and their primary condition upon deposition is horizontal, which this law outlines. The presence of fossils or variation in thickness pertains to different aspects of sedimentology and geological processes, rather than being a direct result of the initial conditions under which the layers were formed. Thus, the correct answer accurately reflects the fundamental concept that sedimentary layers begin their formation in a horizontal orientation.