ABSTRACT: A Pliocene aged, geological unit, which is mostly reddish brown colored, crops out at many places around the Ankara city,. Many geographical locations over the unit are called, with a prefix "kızıl" denoting the characteristic .reddish brown color of the. unit. Therefore the unit is named as Kızıl formation (Plk). There are two common suggestions about the depositional environment of the Plk; — Pik has been deposited by meandering and. braided, rivers. — Plk has deposited in intermountain basins (ponds-lakes). In order to gather geological data, and to find, more evidences,, six distinguishable Pliocene Basins, around Ankara were studied. Identification of the PIk`s depositional environment in datait, can. provide valuable information to implement effective investigation, methods to carry out geotechnical properties of the Plk at any study .area in this unit., Naturally» engineering properli.es of the unit depends mainly upon the position and. location of any delineated site. Grain, size, composition, thickness of layers, areal extent, interested depth, distance, from paleoshore, and. catchment, area properties, are main factors which control geotechnical properties of the unit at a given site. The Plk has deposited over a hummocky topography. Its basal level elevation on the same line along short axis of Hatip ovası basin, changes from > 1200 m to < 700 m. It consists mainly of sedimentary breccia, conglomerate, and sandstone along border zone whereas siltstone, muds tone, and claystone along longitudinal axis of basins.. Sorting from shore to the deepest part is quite common in the Pliocene basins .around Ankara.Main cement type in the unit is CH clay Consequently rock  structure breaks easily and partly turns into soil where it has been fully saturated for long period.However bedding planes remain distinct. This situation is observable mainly along long axes of basins Moderately weak to moderately strong sedimentary rocks, along border zone, gradual ly transits into extremely weak sedimentary rocks toward the deepest part of a basin In addition to the high CH clay content and saturation,, stress relief is well effective on engineering properties of the unit. Therefore the uppermost level of the unit, especially over a gently sloping and .relatively wide area whichs includes thick pebbly soil mantle derived from the Plk, is called Pebbly soil member (Plkp).Evaluation of numereous field investigation and laboratory works support basically the second suggestion which says that the Plk. has deposited, in intermountain basins. It is difficult to find geological, evidences to support, the proposal saying the Plk has formed as stream, deposits.