Glacial diamicts, such as clayey tills and sediment gravity flows, are common all over theNorthern Hemisphere due to the many Pleistocene (2.58 Ma - 11.7 ka) glaciations. Thus, de-tailed geological characterisation of glacial diamicts is important to solve a wide range ofchallenges important to society; such as groundwater flow and contaminant transport, con-struction of building and infrastructure, as well as drainage systems related to climate changeadaptations.However, glacial deposits are usually highly heterogeneous both laterally and verticallydue to the dynamic nature of glacial depositional environments and glaciotectonics. It cantherefore be difficult to establish reliable geological models from boreholes alone and a mini-mally invasive method is therefore needed.This thesis therefore investigates whether crosshole ground-penetrating radar (GPR) canbe used for geological characterisation of clayey glacial diamicts, as no off-the-shelf geo-physical method currently exists for investigating the decimetre- to metre-scale variation ofthese deposits. The motivation for focusing on clayey materials is two-fold: Firstly, the pres-ence of clay has a strong impact on the aforementioned challenges. Secondly, propagation ofradar waves through clayey materials in the 1-300 MHz band, necessary for geological stud-ies, is poorly described.By combining geological and geophysical field work at a hummocky moraine site nearHolbæk, Denmark with numerical full-waveform studies of radar wave propagation throughdispersive materials, we were able to describe a GPR signal’s speed, strength and shape re-sponse to clayey diamicts and other fined-grained materials. Overall, we find that the radarvelocity is controlled by the water content, and that the changes in amplitude and shape areprimarily controlled by changes in grain size. Most importantly, we find that these differentradar responses can be used to derive 3D geological information about the metre-scale heter-ogeneity in glacial diamict deposits.