Lithologic Logs of Observation Wells and Test Holes Drilled in 1987, Fort Belknap Indian Reservation

By Wyoming-Montana Water Science Center

This page summarizes lithologic logs from 66 observation wells and 44 test holes drilled in 1987 in valley fill along the north flank of the Little Rocky Mountains at the Fort Belknap Indian Reservation. The drilling supported a water-resources investigation to define the thickness and extent of valley-fill materials. Geologic units identified range in age from Jurassic to Holocene.

Open Report

Objective

The main goal was to determine the geometry (thickness and lateral distribution) of valley-fill materials and assess the potential for groundwater withdrawals for irrigation, given inadequate surface-water supply in the area.

Findings

The logs cover geologic units ranging from Jurassic to Holocene. The principal aquifer is layered sand and gravel in the lower part of the valley-fill sequence. Glacial deposits also yield water to wells, while terrace deposits and colluvium generally are not aquifers. Water-yielding properties are summarized for each unit, with yields ranging from a few to several hundred gallons per minute depending on the material and location.

Publication

The report includes tables of lithologic logs for each well and test hole, describing the materials, thicknesses, and water production characteristics. Maps show the location of the study area, wells, and test holes. Tables provide detailed descriptions of geologic units, their water-yielding properties, and the lithologic logs for each site.