Iowa Depth to Bedrock

Map Description

This depth-to-bedrock map estimates the physical boundary between regolith (unconsolidated) and hard rock (consolidated) materials, which can influence rooting depth, water storage, drainage behavior, land productivity, and the costs of digging to different depths. In Iowa’s loess and till landscapes, a depth-to-bedrock map is especially valuable because it distinguishes areas with thick loess or glacial sediments from those where bedrock lies relatively close to the surface. In certain physiographic regions or where incised stream valleys exhume the underlying bedrock, the depth to consolidated material can be within tens of centimeters/inches of the soil surface.

Shallow bedrock limits root growth, promotes perched water tables, leading to excess wetness, and reduces available water, increasing drought sensitivity. At the watershed scale, bedrock depth governs how water is partitioned among runoff, shallow subsurface flow, and deep subsurface flow. Those factors shape stream response to rainfall events and nutrient transport. Hydrologic pathways impact patterns of soil formation and long-term erosion risk. For engineering and land-use decisions, bedrock depth informs foundation design, drainage feasibility, and construction cost.

GIS Process

This depth-to-bedrock map was produced by subtracting the estimated bedrock surface elevation from the ground surface elevation derived from LiDAR. The Iowa Geological Survey (IGS) provides a statewide bedrock surface elevation model, constructed from well logs, outcrop observations, geophysical data, and stratigraphic interpretation, which represents the elevation of the top of consolidated bedrock beneath unconsolidated sediments such as loess, alluvium, and till. The IGS model of bedrock surface elevation was mapped at a 110 m spatial resolution. LiDAR elevation data for the ground surface is naturally much more accurate and has a finer spatial resolution. To balance not throwing out too much detailed information with the uncertainty introduced by matching datasets with disparate spatial resolutions, the 30 m resolution bare-earth surface DEM was used.