What does "very gravely" soil contain by volume?

The classification of "very gravely" soil is based on its composition by volume, specifically concerning the presence of rock fragments. In "very gravely" soil, there is a significant amount of rock fragments, which is defined as being between 35% and 60% by volume. This means that when evaluating the soil's texture, the volume of rock fragments is notably high, influencing its drainage and stability characteristics.

Understanding the composition of soil is essential for septic system design and installation, as it directly affects water movement and filtration capabilities. The high content of rock fragments in very gravely soil can lead to rapid drainage, making it an important consideration for proper septic system placement to avoid issues such as ineffective absorption. The other choices represent different classifications of soil based on rock fragment volume that do not meet the criteria for "very gravely," thus not fitting the definition accurately.