Unlike Bitcoin, the block header that is hashed for PoW in XMR doesn't seem to be the same as the block header structure in the code. In other words, this header structure seems exclusively used for PoW. Its format is as follows:

Field Description Size (bytes)
major_version Block major version number Variable
minor_version Block minor version number Variable
timestamp Ditto Variable
prev_id Hash of previous block (NOT the PoW hash) 32
nonce 32-bit number used when mining 4
tx_hashes Merkle root 32
txcount Number of transactions included in the block Variable

The variable sized integers are stored in Monero's varint format.