Pratt's Theory, proposed by the American geologist John P. Pratt in the late 19th century, suggests that the Earth's crust is not uniform but varies in thickness and density. This theory explains how different landforms, such as mountains and valleys, are supported by varying densities of the underlying materials. According to Pratt, areas with lighter, less dense materials can support higher elevations, while denser materials are found in lower regions. This concept helps geologists understand the distribution of landforms and the processes that shape the Earth's surface over time, contributing to the field of isostasy.