The rain filters and impregnates the soil, but not being able to soak beyond a certain depth, on account of the volcanic rocks of the undersoil, forms a small stratum always met with at a certain depth.
"A Narrative of Captivity in Abyssinia With Some Account of the Late Emperor Theodore, His Country and People"
Henry Blanc
Warm as the weather had been at Fort Grey, the night was bitterly cold, with the wind from the S.S.E. We left this, our first well, at early dawn, riding across a continuation of the same grassy and sandy land as that we had journeyed over the day before, only that it had many bare patches upon it full of water, the undersoil being a red clay.
"Expedition into Central Australia"
Charles Sturt
Fortunately the undersoil was stiff, the sides of the trenches could be cut quite perpendicular and in fine weather there was slight risk of the under-cutting causing subsidences.
"The Fifth Battalion Highland Light Infantry in the War 1914-1918"
F.L. Morrison