Secondary compression is one of the fundamental behaviors exhibited by geologic materials. It has direct effects on the state of stress, compression, and the strength of materials in situ. We studied the secondary compression behavior of one smectitic, one illitic, and one kaolinitic mudrock in 1 to 100 MPa stress range under one-dimensional strain conditions. We tested both intact and laboratory resedimented specimens and found that their behaviors agree well in terms of both compression due to stress and secondary compression. We show that the secondary compression index, C_α, decreases or stays constant with time and stress in the normally consolidated state and generally scales with the compression index, C_c, depending on the material. The ratio of C_α to C_c is found to be within the range of 1.5 to 3.5 %. We present a more objective method to interpret the end of primary consolidation and C_α for soils that exhibit decreasing rate of secondary compression with time. We did not observe a trend between the secondary compression index and plasticity of the clay. We conclude that the secondary compression behavior at geological stress levels is aligned with the behavior at low stresses.