Why Clk Oscillator waveform is not square in the oscilloscope?

Thank you for information. My FPGA board is Cyclone II and it has three clock 50 Mhz, 27 Mhz and 24 Mhz. I am using the 50 Mhz clock now and I want to see the clock signal (normally it should be as a square waveform) but it is not square. Also as you said I am going to check the scope and probes.