We still have a very limited knowledge about the formation of the early Milky Way. A line of research has focused on understanding the nature of the stellar halo, finding kinematical and chemical evidence of substructure. The existence of substructure is in line with the hierarchical formation scenario predicted by ΛCDM simulations for large galaxies such as ours. Studies based on high resolution spectroscopy have been limited to stars in the solar neighbourhood, and their conclusions rely on correlations between kinematics and chemistry. In this work, we have searched for halo stars in situ, using mid-resolution ( 2000) spectra included in the SDSS/SEGUE survey for stars at distances between 5 kpc and 100 kpc. We have analyzed a sample of stars with effective temperatures in the range 5800 < Teff < 6300 K, surface gravities between 0.0 < logg < 5.0 dex and metallicities between -2.0 < [Fe/H] < -0.4 dex. We have derived Ca/Fe and Mg/Fe abundance ratios from the spectra and found clear correlations with metallicity and distance from the Galactic center.