An apparatus for drilling a well which includes a drill bit (2) arranged at the end of a length of drill tubing (4), a motor (3) to rotate the drill bit and steering means to steer the drill bit, and including torque measuring means (5) to measure the torque applied to the drill bit continuously and processing means to calculate values for the mechanical specific energy (MSB) and measured depth data over time whilst drilling. The processing means includes comparison means which is configured to compare the measured data with known data to determine the nature of the formation (6) being drilled compared to known types of formation, and which processing means is configured to indicate a change from a first formation type to a second formation type, thus indicating the presence of a formation boundary, when the drill bit is adjacent to or just past the formation boundary (7).